Switch to parking_lot::{Mutex, RwLock}

Switch to the `parking_lot` crate's implementations of
`std::sync::Mutex` and `std::sync::RwLock`, which are more efficient.

A writeup on why `parking_lot` is more efficient can be read here:
<https://github.com/Amanieu/parking_lot>

Upgrade path:

Modify `mutex.lock().unwrap()` usage to `mutex.lock()` (not needing to
unwrap or handle a result), and
`rwlock.read().unwrap()`/`rwlock.write().unwrap()` usage to
`rwlock.read()` and `rwlock.write()`.

For example, modify:

```rust
use serenity::CACHE;

println!("{}", CACHE.read().unwrap().user.id);
```

to:

```rust
use serenity::CACHE;

println!("{}", CACHE.read().user.id);
```

1 files changed, 2 insertions, 2 deletions

diff --git a/src/model/guild/guild_id.rs b/src/model/guild/guild_id.rs
index 4b9a713..efda6a8 100644
--- a/src/model/guild/guild_id.rs
+++ b/src/model/guild/guild_id.rs
@@ -305,7 +305,7 @@ impl GuildId {
 
     /// Search the cache for the guild.
     #[cfg(feature = "cache")]
-    pub fn find(&self) -> Option<Arc<RwLock<Guild>>> { CACHE.read().unwrap().guild(*self) }
+    pub fn find(&self) -> Option<Arc<RwLock<Guild>>> { CACHE.read().guild(*self) }
 
     /// Requests the guild over REST.
     ///
@@ -408,7 +408,7 @@ impl GuildId {
     /// [`utils::shard_id`]: ../utils/fn.shard_id.html
     #[cfg(all(feature = "cache", feature = "utils"))]
     #[inline]
-    pub fn shard_id(&self) -> u64 { ::utils::shard_id(self.0, CACHE.read().unwrap().shard_count) }
+    pub fn shard_id(&self) -> u64 { ::utils::shard_id(self.0, CACHE.read().shard_count) }
 
     /// Returns the Id of the shard associated with the guild.
     ///