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, 8 insertions, 7 deletions

diff --git a/src/builder/create_embed.rs b/src/builder/create_embed.rs
index ae0975b..1b15253 100644
--- a/src/builder/create_embed.rs
+++ b/src/builder/create_embed.rs
@@ -17,7 +17,7 @@
 
 use chrono::{DateTime, TimeZone};
 use internal::prelude::*;
-use model::Embed;
+use model::channel::Embed;
 use serde_json::Value;
 use std::collections::HashMap;
 use std::default::Default;
@@ -203,9 +203,9 @@ impl CreateEmbed {
     /// Passing a string timestamp:
     ///
     /// ```rust,no_run
-    /// # use serenity::prelude::*;
-    /// # use serenity::model::*;
-    /// #
+    /// use serenity::prelude::*;
+    /// use serenity::model::channel::Message;
+    ///
     /// struct Handler;
     ///
     /// impl EventHandler for Handler {
@@ -229,9 +229,10 @@ impl CreateEmbed {
     /// Note: this example isn't efficient and is for demonstrative purposes.
     ///
     /// ```rust,no_run
-    /// # use serenity::prelude::*;
-    /// # use serenity::model::*;
-    /// #
+    /// use serenity::prelude::*;
+    /// use serenity::model::guild::Member;
+    /// use serenity::model::id::GuildId;
+    ///
     /// struct Handler;
     ///
     /// impl EventHandler for Handler {