sync: Update the arc module

This removes the now-outdated MutexArc and RWArc types. These are superseded by
Arc<Mutex<T>> and Arc<RWLock<T>>. The only remaining arc is the one true Arc.
Additionally, the arc now has weak pointers implemented for it to assist in
breaking cycles.

This commit brings the arc api up to parity with the sibling Rc api, making them
nearly interchangeable for inter and intra task communication.

Author: alexcrichton