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/builder/get_messages.rs b/src/builder/get_messages.rs
index e59584f..1032a2f 100644
--- a/src/builder/get_messages.rs
+++ b/src/builder/get_messages.rs
@@ -1,4 +1,4 @@
-use model::MessageId;
+use model::id::MessageId;
 use std::collections::HashMap;
 
 /// Builds a request for a request to the API to retrieve messages.
@@ -29,8 +29,8 @@ use std::collections::HashMap;
 /// # use std::error::Error;
 /// #
 /// # fn try_main() -> Result<(), Box<Error>> {
-/// use serenity::model::{ChannelId, MessageId};
 /// use serenity::builder::GetMessages;
+/// use serenity::model::id::{ChannelId, MessageId};
 ///
 /// let retriever = GetMessages::default()
 ///     .after(MessageId(158339864557912064))