Break up the model module

The `model` module has historically been one giant module re-exporting
all of the model types, which is somewhere around 100 types. This can be
a lot to look at for a new user and somewhat overwhelming, especially
with a large number of fine-grained imports from the module.

The module is now neatly split up into submodules, mostly like it has
been internally since the early versions of the library. The submodules
are:

- application
- channel
- error
- event
- gateway
- guild
- id
- invite
- misc
- permissions
- prelude
- user
- voice
- webhook

Each submodule contains types that are "owned" by the module. For
example, the `guild` submodule contains, but not limited to, Emoji,
AuditLogsEntry, Role, and Member. `channel` contains, but not limited
to, Attachment, Embed, Message, and Reaction.

Upgrade path:

Instead of glob importing the models via `use serenity::model::*;`,
instead glob import via the prelude:

```rust
use serenity::model::prelude::*;
```

Instead of importing from the root model module:

```rust
use serenity::model::{Guild, Message, OnlineStatus, Role, User};
```

instead import from the submodules like so:

```rust
use serenity::model::channel::Message;
use serenity::model::guild::{Guild, Role};
use serenity::model::user::{OnlineStatus, User};
```

10 files changed, 33 insertions, 34 deletions

diff --git a/src/model/channel/attachment.rs b/src/model/channel/attachment.rs
index 1836498..a9c625b 100644
--- a/src/model/channel/attachment.rs
+++ b/src/model/channel/attachment.rs
@@ -43,8 +43,8 @@ impl Attachment {
     /// Download all of the attachments associated with a [`Message`]:
     ///
     /// ```rust,no_run
+    /// use serenity::model::prelude::*;
     /// use serenity::prelude::*;
-    /// use serenity::model::*;
     /// use std::env;
     /// use std::fs::File;
     /// use std::io::Write;
@@ -52,7 +52,6 @@ impl Attachment {
     ///
     /// struct Handler;
     ///
-    ///
     /// impl EventHandler for Handler {
     ///     fn message(&self, _: Context, message: Message) {
     ///         for attachment in message.attachments {
diff --git a/src/model/channel/channel_category.rs b/src/model/channel/channel_category.rs
index 73c50a7..f6df342 100644
--- a/src/model/channel/channel_category.rs
+++ b/src/model/channel/channel_category.rs
@@ -1,4 +1,4 @@
-use model::*;
+use model::prelude::*;
 
 #[cfg(all(feature = "builder", feature = "model"))]
 use builder::EditChannel;
diff --git a/src/model/channel/channel_id.rs b/src/model/channel/channel_id.rs
index acd3704..7ec6dfe 100644
--- a/src/model/channel/channel_id.rs
+++ b/src/model/channel/channel_id.rs
@@ -1,5 +1,5 @@
 use internal::RwLockExt;
-use model::*;
+use model::prelude::*;
 
 #[cfg(feature = "model")]
 use std::borrow::Cow;
@@ -408,7 +408,7 @@ impl ChannelId {
     /// Send files with the paths `/path/to/file.jpg` and `/path/to/file2.jpg`:
     ///
     /// ```rust,no_run
-    /// use serenity::model::ChannelId;
+    /// use serenity::model::id::ChannelId;
     ///
     /// let channel_id = ChannelId(7);
     ///
@@ -420,7 +420,7 @@ impl ChannelId {
     /// Send files using `File`:
     ///
     /// ```rust,no_run
-    /// use serenity::model::ChannelId;
+    /// use serenity::model::id::ChannelId;
     /// use std::fs::File;
     ///
     /// let channel_id = ChannelId(7);
diff --git a/src/model/channel/embed.rs b/src/model/channel/embed.rs
index a839beb..3cbe5ac 100644
--- a/src/model/channel/embed.rs
+++ b/src/model/channel/embed.rs
@@ -78,7 +78,7 @@ impl Embed {
     /// Create an embed:
     ///
     /// ```rust,no_run
-    /// use serenity::model::Embed;
+    /// use serenity::model::channel::Embed;
     ///
     /// let embed = Embed::fake(|e| e
     ///     .title("Embed title")
diff --git a/src/model/channel/group.rs b/src/model/channel/group.rs
index dfdfd0f..bff58aa 100644
--- a/src/model/channel/group.rs
+++ b/src/model/channel/group.rs
@@ -1,5 +1,5 @@
 use chrono::{DateTime, FixedOffset};
-use model::*;
+use model::prelude::*;
 
 #[cfg(feature = "model")]
 use builder::{CreateMessage, GetMessages};
diff --git a/src/model/channel/guild_channel.rs b/src/model/channel/guild_channel.rs
index 54f802d..9b8371d 100644
--- a/src/model/channel/guild_channel.rs
+++ b/src/model/channel/guild_channel.rs
@@ -1,5 +1,5 @@
 use chrono::{DateTime, FixedOffset};
-use model::*;
+use model::prelude::*;
 
 #[cfg(all(feature = "cache", feature = "model"))]
 use CACHE;
@@ -137,18 +137,17 @@ impl GuildChannel {
     /// permissions:
     ///
     /// ```rust,no_run
-    /// # use serenity::model::{ChannelId, Permissions, UserId};
+    /// # use serenity::model::id::{ChannelId, UserId};
     /// # use std::error::Error;
     /// #
     /// # fn try_main() -> Result<(), Box<Error>> {
     /// #     let (channel_id, user_id) = (ChannelId(0), UserId(0));
     /// #
-    /// use serenity::model::{
-    ///     ModelError,
+    /// use serenity::model::channel::{
     ///     PermissionOverwrite,
     ///     PermissionOverwriteType,
-    ///     permissions,
     /// };
+    /// use serenity::model::{ModelError, Permissions};
     /// use serenity::CACHE;
     ///
     /// let allow = Permissions::SEND_MESSAGES;
@@ -179,18 +178,17 @@ impl GuildChannel {
     /// permissions:
     ///
     /// ```rust,no_run
-    /// # use serenity::model::{ChannelId, Permissions, UserId};
+    /// # use serenity::model::id::{ChannelId, UserId};
     /// # use std::error::Error;
     /// #
     /// # fn try_main() -> Result<(), Box<Error>> {
     /// #     let (channel_id, user_id) = (ChannelId(0), UserId(0));
     /// #
-    /// use serenity::model::{
-    ///     ModelError,
+    /// use serenity::model::channel::{
     ///     PermissionOverwrite,
     ///     PermissionOverwriteType,
-    ///     permissions,
     /// };
+    /// use serenity::model::{ModelError, Permissions};
     /// use serenity::CACHE;
     ///
     /// let allow = Permissions::SEND_MESSAGES;
@@ -425,7 +423,7 @@ impl GuildChannel {
     ///
     /// ```rust,no_run
     /// use serenity::prelude::*;
-    /// use serenity::model::*;
+    /// use serenity::model::prelude::*;
     /// struct Handler;
     ///
     /// use serenity::CACHE;
@@ -452,10 +450,9 @@ impl GuildChannel {
     /// for demonstrative purposes):
     ///
     /// ```rust,no_run
-    /// use serenity::prelude::*;
-    /// use serenity::model::*;
     /// use serenity::CACHE;
-    /// use serenity::model::permissions;
+    /// use serenity::prelude::*;
+    /// use serenity::model::prelude::*;
     /// use std::fs::File;
     ///
     /// struct Handler;
@@ -468,11 +465,10 @@ impl GuildChannel {
     ///         };
     ///
     ///         let current_user_id = CACHE.read().user.id;
-    /// let permissions =
-    /// channel.read().permissions_for(current_user_id).unwrap();
+    ///         let permissions =
+    ///             channel.read().permissions_for(current_user_id).unwrap();
     ///
-    /// if !permissions.contains(Permissions::ATTACH_FILES |
-    /// Permissions::SEND_MESSAGES) {
+    ///         if !permissions.contains(Permissions::ATTACH_FILES | Permissions::SEND_MESSAGES) {
     ///             return;
     ///         }
     ///
@@ -485,8 +481,8 @@ impl GuildChannel {
     ///             },
     ///         };
     ///
-    /// let _ = msg.channel_id.send_files(vec![(&file, "cat.png")], |m|
-    /// m.content("here's a cat"));
+    ///         let _ = msg.channel_id.send_files(vec![(&file, "cat.png")], |m|
+    ///             m.content("here's a cat"));
     ///     }
     /// }
     ///
diff --git a/src/model/channel/message.rs b/src/model/channel/message.rs
index 65df51a..f139aab 100644
--- a/src/model/channel/message.rs
+++ b/src/model/channel/message.rs
@@ -1,5 +1,7 @@
+//! Models relating to Discord channels.
+
 use chrono::{DateTime, FixedOffset};
-use model::*;
+use model::prelude::*;
 use serde_json::Value;
 
 #[cfg(feature = "model")]
@@ -85,7 +87,7 @@ impl Message {
     /// #   impl EventHandler for Handler {}
     /// #   let mut client = Client::new("token", Handler).unwrap();
     /// #
-    /// use serenity::model::Channel;
+    /// use serenity::model::channel::Channel;
     /// use serenity::framework::StandardFramework;
     ///
     /// client.with_framework(StandardFramework::new()
diff --git a/src/model/channel/mod.rs b/src/model/channel/mod.rs
index 2fc4cbc..479818b 100644
--- a/src/model/channel/mod.rs
+++ b/src/model/channel/mod.rs
@@ -1,3 +1,5 @@
+//! Models relating to channels and types within channels.
+
 mod attachment;
 mod channel_id;
 mod embed;
@@ -19,7 +21,7 @@ pub use self::reaction::*;
 pub use self::channel_category::*;
 
 use internal::RwLockExt;
-use model::*;
+use model::prelude::*;
 use serde::de::Error as DeError;
 use serde_json;
 use super::utils::deserialize_u64;
diff --git a/src/model/channel/private_channel.rs b/src/model/channel/private_channel.rs
index a05b750..c9b6524 100644
--- a/src/model/channel/private_channel.rs
+++ b/src/model/channel/private_channel.rs
@@ -1,5 +1,5 @@
 use chrono::{DateTime, FixedOffset};
-use model::*;
+use model::prelude::*;
 use std::fmt::{Display, Formatter, Result as FmtResult};
 use super::deserialize_single_recipient;
 
diff --git a/src/model/channel/reaction.rs b/src/model/channel/reaction.rs
index b2fdffd..5c1ee4d 100644
--- a/src/model/channel/reaction.rs
+++ b/src/model/channel/reaction.rs
@@ -1,4 +1,4 @@
-use model::*;
+use model::prelude::*;
 use serde::de::{Deserialize, Error as DeError, MapAccess, Visitor};
 use std::error::Error as StdError;
 use std::fmt::{Display, Formatter, Result as FmtResult, Write as FmtWrite};
@@ -270,7 +270,7 @@ impl From<char> for ReactionType {
     /// Reacting to a message with an apple:
     ///
     /// ```rust,no_run
-    /// # use serenity::model::ChannelId;
+    /// # use serenity::model::id::ChannelId;
     /// # use std::error::Error;
     /// #
     /// # fn try_main() -> Result<(), Box<Error>> {
@@ -327,7 +327,7 @@ impl<'a> From<&'a str> for ReactionType {
     /// rest of the library:
     ///
     /// ```rust
-    /// use serenity::model::ReactionType;
+    /// use serenity::model::channel::ReactionType;
     ///
     /// fn foo<R: Into<ReactionType>>(bar: R) {
     ///     println!("{:?}", bar.into());