sync: Bring the library up to date

* Remove clone-ability from all primitives. All shared state will now come
  from the usage of the primitives being shared, not the primitives being
  inherently shareable. This allows for fewer allocations for stack-allocated
  primitives.
* Add `Mutex<T>` and `RWLock<T>` which are stack-allocated primitives for purely
  wrapping a piece of data
* Remove `RWArc<T>` in favor of `Arc<RWLock<T>>`
* Remove `MutexArc<T>` in favor of `Arc<Mutex<T>>`
* Shuffle around where things are located
  * The `arc` module now only contains `Arc`
  * A new `lock` module contains `Mutex`, `RWLock`, and `Barrier`
  * A new `raw` module contains the primitive implementations of `Semaphore`,
    `Mutex`, and `RWLock`
* The Deref/DerefMut trait was implemented where appropriate
* `CowArc` was removed, the functionality is now part of `Arc` and is tagged
  with `#[experimental]`.
* The crate now has #[deny(missing_doc)]
* `Arc` now supports weak pointers

This is not a large-scale rewrite of the functionality contained within the
`sync` crate, but rather a shuffling of who does what an a thinner hierarchy of
ownership to allow for better composability.