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, 9 insertions, 6 deletions

diff --git a/src/gateway/mod.rs b/src/gateway/mod.rs
index c2acaac..7e4e2b0 100644
--- a/src/gateway/mod.rs
+++ b/src/gateway/mod.rs
@@ -57,7 +57,8 @@ pub use self::error::Error as GatewayError;
 pub use self::shard::Shard;
 pub use self::ws_client_ext::WebSocketGatewayClientExt;
 
-use model::{Game, OnlineStatus};
+use model::gateway::Game;
+use model::user::OnlineStatus;
 use websocket::sync::client::Client;
 use websocket::sync::stream::{TcpStream, TlsStream};
 
diff --git a/src/gateway/shard.rs b/src/gateway/shard.rs
index 333d693..c00affc 100644
--- a/src/gateway/shard.rs
+++ b/src/gateway/shard.rs
@@ -1,7 +1,9 @@
 use constants::{self, close_codes};
 use internal::prelude::*;
 use model::event::{Event, GatewayEvent};
-use model::{Game, GuildId, OnlineStatus};
+use model::gateway::Game;
+use model::id::GuildId;
+use model::user::OnlineStatus;
 use parking_lot::Mutex;
 use std::sync::Arc;
 use std::time::{Duration as StdDuration, Instant};
@@ -297,7 +299,7 @@ impl Shard {
     /// #
     /// # 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")));
     /// # }
@@ -755,7 +757,7 @@ impl Shard {
     /// #
     /// #     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)];
     ///
@@ -785,7 +787,7 @@ impl Shard {
     /// #
     /// #     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)];
     ///
diff --git a/src/gateway/ws_client_ext.rs b/src/gateway/ws_client_ext.rs
index 873d114..934383c 100644
--- a/src/gateway/ws_client_ext.rs
+++ b/src/gateway/ws_client_ext.rs
@@ -3,7 +3,7 @@ use constants::{self, OpCode};
 use gateway::{CurrentPresence, WsClient};
 use internal::prelude::*;
 use internal::ws_impl::SenderExt;
-use model::GuildId;
+use model::id::GuildId;
 use std::env::consts;
 
 pub trait WebSocketGatewayClientExt {