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, 9 insertions, 5 deletions

diff --git a/src/model/guild/emoji.rs b/src/model/guild/emoji.rs
index 31d2a1f..9581f77 100644
--- a/src/model/guild/emoji.rs
+++ b/src/model/guild/emoji.rs
@@ -1,5 +1,5 @@
 use std::fmt::{Display, Formatter, Result as FmtResult, Write as FmtWrite};
-use super::super::{EmojiId, RoleId};
+use super::super::id::{EmojiId, RoleId};
 
 #[cfg(all(feature = "cache", feature = "model"))]
 use internal::prelude::*;
@@ -51,7 +51,8 @@ impl Emoji {
     /// Delete a given emoji:
     ///
     /// ```rust,no_run
-    /// # use serenity::model::{Emoji, EmojiId};
+    /// # use serenity::model::guild::Emoji;
+    /// # use serenity::model::id::EmojiId;
     /// #
     /// # let mut emoji = Emoji {
     /// #     id: EmojiId(7),
@@ -88,7 +89,8 @@ impl Emoji {
     /// Change the name of an emoji:
     ///
     /// ```rust,no_run
-    /// # use serenity::model::{Emoji, EmojiId};
+    /// # use serenity::model::guild::Emoji;
+    /// # use serenity::model::id::EmojiId;
     /// #
     /// # let mut emoji = Emoji {
     /// #     id: EmojiId(7),
@@ -132,7 +134,8 @@ impl Emoji {
     /// Print the guild id that owns this emoji:
     ///
     /// ```rust,no_run
-    /// # use serenity::model::{Emoji, EmojiId};
+    /// # use serenity::model::guild::Emoji;
+    /// # use serenity::model::id::EmojiId;
     /// #
     /// # let mut emoji = Emoji {
     /// #     id: EmojiId(7),
@@ -167,7 +170,8 @@ impl Emoji {
     /// Print the direct link to the given emoji:
     ///
     /// ```rust,no_run
-    /// # use serenity::model::{Emoji, EmojiId};
+    /// # use serenity::model::guild::Emoji;
+    /// # use serenity::model::id::EmojiId;
     /// #
     /// # let mut emoji = Emoji {
     /// #     id: EmojiId(7),