sync: Rewrite the base primitives

This commit rewrites the core primitives of the sync library: Mutex, RWLock, and
Semaphore. These primitives now have updated, more modernized apis:

* Guards are returned instead of locking with closures. All condition variables
  have moved inside the guards and extraneous methods have been removed.
* Downgrading on an rwlock is now done through the guard instead of the rwlock
  itself.

These types are meant to be general locks, not locks of an internal type (for
external usage). New types will be introduced for locking shared data.

Author: alexcrichton