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, 3 deletions

diff --git a/tests/test_msg_builder.rs b/tests/test_msg_builder.rs
index 3062fc5..e4deae4 100644
--- a/tests/test_msg_builder.rs
+++ b/tests/test_msg_builder.rs
@@ -4,9 +4,8 @@ extern crate serenity;
 
 use serenity::utils::MessageBuilder;
 use serenity::utils::ContentModifier::*;
-use serenity::model::Emoji;
-use serenity::model::EmojiId;
-use serenity::model::UserId;
+use serenity::model::guild::Emoji;
+use serenity::model::id::{EmojiId, UserId};
 
 #[test]
 fn code_blocks() {