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, 4 insertions, 4 deletions

diff --git a/src/cache/mod.rs b/src/cache/mod.rs
index 883284d..4f6d476 100644
--- a/src/cache/mod.rs
+++ b/src/cache/mod.rs
@@ -42,7 +42,7 @@
 //! [`CACHE`]: ../struct.CACHE.html
 //! [`http`]: ../http/index.html
 
-use model::*;
+use model::prelude::*;
 use parking_lot::RwLock;
 use std::collections::hash_map::Entry;
 use std::collections::{HashMap, HashSet};
@@ -168,8 +168,8 @@ impl Cache {
     /// used to determine how many members have not yet been received.
     ///
     /// ```rust,no_run
+    /// # use serenity::model::prelude::*;
     /// # use serenity::prelude::*;
-    /// # use serenity::model::*;
     /// #
     /// # #[cfg(feature = "client")]
     /// # fn main() {
@@ -264,8 +264,8 @@ impl Cache {
     /// ```rust,no_run
     /// # #[cfg(feature = "client")]
     /// # fn main() {
+    /// # use serenity::model::prelude::*;
     /// # use serenity::prelude::*;
-    /// # use serenity::model::*;
     /// #
     /// use serenity::CACHE;
     ///
@@ -379,8 +379,8 @@ impl Cache {
     /// ```rust,no_run
     /// # #[cfg(feature = "client")]
     /// # fn main() {
+    /// # use serenity::model::prelude::*;
     /// # use serenity::prelude::*;
-    /// # use serenity::model::*;
     /// #
     /// use serenity::CACHE;
     ///