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};
```

2 files changed, 8 insertions, 6 deletions

diff --git a/src/client/bridge/gateway/shard_messenger.rs b/src/client/bridge/gateway/shard_messenger.rs
index 069c838..a4b63f3 100644
--- a/src/client/bridge/gateway/shard_messenger.rs
+++ b/src/client/bridge/gateway/shard_messenger.rs
@@ -1,4 +1,4 @@
-use model::{Game, GuildId, OnlineStatus};
+use model::prelude::*;
 use super::{ShardClientMessage, ShardRunnerMessage};
 use std::sync::mpsc::{SendError, Sender};
 use websocket::message::OwnedMessage;
@@ -62,7 +62,7 @@ impl ShardMessenger {
     /// #
     /// #     let mut shard = Shard::new(mutex.clone(), mutex, [0, 1])?;
     /// #
-    /// use serenity::model::GuildId;
+    /// use serenity::model::id::GuildId;
     ///
     /// let guild_ids = vec![GuildId(81384788765712384)];
     ///
@@ -92,7 +92,7 @@ impl ShardMessenger {
     /// #
     /// #     let mut shard = Shard::new(mutex.clone(), mutex, [0, 1])?;
     /// #
-    /// use serenity::model::GuildId;
+    /// use serenity::model::id::GuildId;
     ///
     /// let guild_ids = vec![GuildId(81384788765712384)];
     ///
@@ -146,7 +146,7 @@ impl ShardMessenger {
     /// #
     /// #     let mut shard = Shard::new(mutex.clone(), mutex, [0, 1]).unwrap();
     /// #
-    /// use serenity::model::Game;
+    /// use serenity::model::gateway::Game;
     ///
     /// shard.set_game(Some(Game::playing("Heroes of the Storm")));
     /// #     Ok(())
@@ -227,7 +227,7 @@ impl ShardMessenger {
     /// #
     /// #     let mut shard = Shard::new(mutex.clone(), mutex, [0, 1]).unwrap();
     /// #
-    /// use serenity::model::OnlineStatus;
+    /// use serenity::model::user::OnlineStatus;
     ///
     /// shard.set_status(OnlineStatus::DoNotDisturb);
     /// #     Ok(())
diff --git a/src/client/bridge/gateway/shard_runner_message.rs b/src/client/bridge/gateway/shard_runner_message.rs
index e6458eb..5c5512d 100644
--- a/src/client/bridge/gateway/shard_runner_message.rs
+++ b/src/client/bridge/gateway/shard_runner_message.rs
@@ -1,4 +1,6 @@
-use model::{Game, GuildId, OnlineStatus};
+use model::gateway::Game;
+use model::user::OnlineStatus;
+use model::id::GuildId;
 use websocket::message::OwnedMessage;
 
 /// A message to send from a shard over a WebSocket.