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};
```

1 files changed, 8 insertions, 6 deletions

diff --git a/src/model/user.rs b/src/model/user.rs
index 2732f2b..6068da1 100644
--- a/src/model/user.rs
+++ b/src/model/user.rs
@@ -1,7 +1,9 @@
+//! User information-related models.
+
 use serde_json;
 use std::fmt;
 use super::utils::deserialize_u16;
-use super::*;
+use super::prelude::*;
 use internal::prelude::*;
 use model::misc::Mentionable;
 
@@ -159,7 +161,7 @@ impl CurrentUser {
     /// #
     /// # let mut cache = CACHE.write();
     ///
-    /// use serenity::model::permissions::Permissions;
+    /// use serenity::model::Permissions;
     ///
     /// // assuming the cache has been unlocked
     /// let url = match cache.user.invite_url(Permissions::empty()) {
@@ -408,7 +410,7 @@ impl User {
     ///
     /// ```rust,no_run
     /// # use serenity::prelude::*;
-    /// # use serenity::model::*;
+    /// # use serenity::model::prelude::*;
     /// #
     /// use serenity::model::Permissions;
     /// use serenity::CACHE;
@@ -611,7 +613,7 @@ impl User {
     ///
     /// ```rust,no_run
     /// # use serenity::prelude::*;
-    /// # use serenity::model::*;
+    /// # use serenity::model::prelude::*;
     /// #
     /// struct Handler;
     ///
@@ -623,7 +625,7 @@ impl User {
     ///
     /// let mut client = Client::new("token", Handler).unwrap();
     /// #
-    /// use serenity::model::UserId;
+    /// use serenity::model::id::UserId;
     /// use serenity::CACHE;
     /// use std::thread;
     /// use std::time::Duration;
@@ -680,7 +682,7 @@ impl User {
     ///
     /// ```rust,no_run
     /// # use serenity::prelude::*;
-    /// # use serenity::model::*;
+    /// # use serenity::model::prelude::*;
     /// #
     /// use serenity::utils::MessageBuilder;
     /// use serenity::utils::ContentModifier::Bold;