Add a message cache API (#345)

This adds an API for message caching. By default this caches 0 messages per
channel.

This can be customized when instantiating:

```rust
use serenity::cache::{Cache, Settings};

let mut settings = Settings::new();

// Cache 10 messages per channel.
settings.max_messages(10);

let cache = Cache::new_with_settings(settings);
```

After instantiation:

```rust
use serenity::cache::Cache;

let mut cache = Cache::new();
cache.settings_mut().max_messages(10);
```

And during runtime through the global cache:

```rust
use serenity::CACHE;

CACHE.write().settings_mut().max_messages(10);
```

1 files changed, 50 insertions, 0 deletions

diff --git a/src/cache/settings.rs b/src/cache/settings.rs
new file mode 100644
index 0000000..e8e456b
--- /dev/null
+++ b/src/cache/settings.rs
@@ -0,0 +1,50 @@
+/// Settings for the cache.

+///

+/// # Examples

+///

+/// Create new settings, specifying the maximum number of messages:

+///

+/// ```rust

+/// use serenity::cache::Settings as CacheSettings;

+///

+/// let mut settings = CacheSettings::new();

+/// settings.max_messages(10);

+/// ```

+#[derive(Clone, Debug, Default)]

+pub struct Settings {

+    /// The maximum number of messages to store in a channel's message cache.

+    ///

+    /// Defaults to 0.

+    pub max_messages: usize,

+    __nonexhaustive: (),

+}

+

+impl Settings {

+    /// Creates new settings to be used with a cache.

+    #[inline]

+    pub fn new() -> Self {

+        Self::default()

+    }

+

+    /// Sets the maximum number of messages to cache in a channel.

+    ///

+    /// Refer to [`max_messages`] for more information.

+    ///

+    /// # Examples

+    ///

+    /// Set the maximum number of messages to cache:

+    ///

+    /// ```rust

+    /// use serenity::cache::Settings;

+    ///

+    /// let mut settings = Settings::new();

+    /// settings.max_messages(10);

+    /// ```

+    ///

+    /// [`max_messages`]: #structfield.max_messages

+    pub fn max_messages(&mut self, max: usize) -> &mut Self {

+        self.max_messages = max;

+

+        self

+    }

+}