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, 3 insertions, 1 deletions

diff --git a/tests/test_formatters.rs b/tests/test_formatters.rs
index b48c648..fbc60ca 100644
--- a/tests/test_formatters.rs
+++ b/tests/test_formatters.rs
@@ -1,3 +1,4 @@
+extern crate parking_lot;
 extern crate serenity;
 
 use serenity::model::*;
@@ -14,8 +15,9 @@ fn test_formatters() {
 #[cfg(feature = "utils")]
 #[test]
 fn test_mention() {
+    use parking_lot::RwLock;
     use serenity::utils::Colour;
-    use std::sync::{Arc, RwLock};
+    use std::sync::Arc;
 
     let channel = Channel::Guild(Arc::new(RwLock::new(GuildChannel {
         bitrate: None,