Replace std synchronization primitives with parking_lot

[lachlansneff]

parking_lot:0.10.2 is already in the dependency tree, so this shouldn't add any compile-time. parking_lot::Mutex is smaller, generally much faster, and has a more ergonomic api than std::sync::Mutex. Same with RwLock.

[cart]

I'm definitely into the simpler api. I need to do a bit of research into the parking lot Mutex impl, but on paper I like this a lot!

[lachlansneff]

The parking_lot crate is essentially a reimplementation of the WTF Webkit locking library in rust. These libraries reimplement locks as sort of userspace futexes, where there's a global hashmap of address -> lock metastate. In most cases, when there is little contention, the lock just performs an atomic operation on the byte (or word in the case of a RwLock) contained inside it, so performance is very good. In cases of contention, the lock looks up its associated data in the global hashmap and can add itself as a sleeping thread to it.

[cart]

This looks good to go as soon as conflicts are resolved.

[lachlansneff]

@cart Alright, I think this should be good.