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.

This sort of information is useful for creating dynamically cleared caches for use in OS development, for example holding pages of files in memory until the address space is needed for something else.