Add `weak_count` and `strong_count` to Rc and Arc

These functions allow you to see how many weak and strong references
there are to an `Arc`, `Rc`, or an `rc::Weak`. Due to the design of
`Arc` it is not possible to get the number of weak references of an
arbitrary `arc::Weak`. Look in `arc.rs` for a more in-depth explanation.

On `arc::Arc` and `arc::Weak` these operations are wait-free and atomic.