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, 1 deletions

diff --git a/src/model/gateway.rs b/src/model/gateway.rs
index 4edc20e..ca846e8 100644
--- a/src/model/gateway.rs
+++ b/src/model/gateway.rs
@@ -1,6 +1,7 @@
+use parking_lot::RwLock;
 use serde::de::Error as DeError;
 use serde_json;
-use std::sync::{Arc, RwLock};
+use std::sync::Arc;
 use super::utils::*;
 use super::*;