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};
```

3 files changed, 17 insertions, 9 deletions

diff --git a/src/utils/colour.rs b/src/utils/colour.rs
index 4e612cc..c59f4be 100644
--- a/src/utils/colour.rs
+++ b/src/utils/colour.rs
@@ -28,8 +28,9 @@ macro_rules! colour {
 /// via [`g`]:
 ///
 /// ```rust
-/// # use serenity::model::{Role, RoleId, permissions};
-/// use serenity::utils::Colour;
+/// # use serenity::model::guild::Role;
+/// # use serenity::model::id::RoleId;
+/// # use serenity::model::permissions;
 /// #
 /// # let role = Role {
 /// #     colour: Colour::blurple(),
@@ -41,6 +42,8 @@ macro_rules! colour {
 /// #     permissions: permissions::PRESET_GENERAL,
 /// #     position: 7,
 /// # };
+/// #
+/// use serenity::utils::Colour;
 ///
 /// // assuming a `role` has already been bound
 ///
diff --git a/src/utils/message_builder.rs b/src/utils/message_builder.rs
index 79b6123..f94b3fd 100644
--- a/src/utils/message_builder.rs
+++ b/src/utils/message_builder.rs
@@ -1,4 +1,6 @@
-use model::{ChannelId, Emoji, Mentionable, RoleId, UserId};
+use model::guild::Emoji;
+use model::id::{ChannelId, RoleId, UserId};
+use model::misc::Mentionable;
 use std::default::Default;
 use std::fmt::{self, Display, Write};
 use std::ops::Add;
@@ -14,7 +16,7 @@ use std::ops::Add;
 /// value:
 ///
 /// ```rust,no_run
-/// # use serenity::model::{Emoji, EmojiId, UserId};
+/// # use serenity::model::prelude::*;
 /// #
 /// # let user = UserId(1);
 /// # let emoji = Emoji {
@@ -68,7 +70,7 @@ impl MessageBuilder {
     /// and finally building it to retrieve the inner String:
     ///
     /// ```rust
-    /// use serenity::model::ChannelId;
+    /// use serenity::model::id::ChannelId;
     /// use serenity::utils::MessageBuilder;
     ///
     /// let channel_id = ChannelId(81384788765712384);
@@ -105,7 +107,7 @@ impl MessageBuilder {
     /// Mentioning a [`Channel`] by Id:
     ///
     /// ```rust
-    /// use serenity::model::ChannelId;
+    /// use serenity::model::id::ChannelId;
     /// use serenity::utils::MessageBuilder;
     ///
     /// let channel_id = ChannelId(81384788765712384);
@@ -138,7 +140,8 @@ impl MessageBuilder {
     /// Mention an emoji in a message's content:
     ///
     /// ```rust
-    /// use serenity::model::{Emoji, EmojiId};
+    /// use serenity::model::guild::Emoji;
+    /// use serenity::model::id::EmojiId;
     /// use serenity::utils::MessageBuilder;
     ///
     /// let emoji = Emoji {
diff --git a/src/utils/mod.rs b/src/utils/mod.rs
index 6291f81..2660798 100644
--- a/src/utils/mod.rs
+++ b/src/utils/mod.rs
@@ -13,7 +13,8 @@ pub use super::builder;
 
 use base64;
 use internal::prelude::*;
-use model::{EmojiId, EmojiIdentifier};
+use model::id::EmojiId;
+use model::misc::EmojiIdentifier;
 use std::collections::HashMap;
 use std::ffi::OsStr;
 use std::fs::File;
@@ -267,7 +268,8 @@ pub fn parse_channel(mention: &str) -> Option<u64> {
 /// Ensure that a valid [`Emoji`] usage is correctly parsed:
 ///
 /// ```rust
-/// use serenity::model::{EmojiId, EmojiIdentifier};
+/// use serenity::model::id::{EmojiId, GuildId};
+/// use serenity::model::misc::EmojiIdentifier;
 /// use serenity::utils::parse_emoji;
 ///
 /// let expected = EmojiIdentifier {