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, 2 insertions, 2 deletions

diff --git a/src/framework/mod.rs b/src/framework/mod.rs
index c5ddb04..9f72dc6 100644
--- a/src/framework/mod.rs
+++ b/src/framework/mod.rs
@@ -61,11 +61,11 @@ pub mod standard;
 pub use self::standard::StandardFramework;
 
 use client::Context;
-use model::Message;
+use model::channel::Message;
 use threadpool::ThreadPool;
 
 #[cfg(feature = "standard_framework")]
-use model::UserId;
+use model::id::UserId;
 
 /// This trait allows for serenity to either use its builtin framework, or yours.
 pub trait Framework {