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, 6 insertions, 4 deletions

diff --git a/src/model/guild/partial_guild.rs b/src/model/guild/partial_guild.rs
index 857ef0d..d5312b5 100644
--- a/src/model/guild/partial_guild.rs
+++ b/src/model/guild/partial_guild.rs
@@ -1,4 +1,4 @@
-use model::*;
+use model::prelude::*;
 use super::super::utils::{deserialize_emojis, deserialize_roles};
 
 #[cfg(feature = "model")]
@@ -448,7 +448,7 @@ impl PartialGuild {
     /// Obtain a reference to a [`Role`] by its name.
     ///
     /// ```rust,no_run
-    /// use serenity::model::*;
+    /// use serenity::model::prelude::*;
     /// use serenity::prelude::*;
     ///
     /// struct Handler;
@@ -457,8 +457,10 @@ impl PartialGuild {
     ///
     /// impl EventHandler for Handler {
     ///     fn message(&self, _: Context, msg: Message) {
-    ///         if let Some(role) =
-    ///            msg.guild_id().unwrap().get().unwrap().role_by_name("role_name") {
+    ///         let guild = msg.guild_id().unwrap().get().unwrap();
+    ///         let possible_role = guild.role_by_name("role_name");
+    ///
+    ///         if let Some(role) = possible_role {
     ///             println!("Obtained role's reference: {:?}", role);
     ///         }
     ///     }