Stabilize cell

This patch finalizes stabilization for the `cell` module, settling on
the current names `Cell`, `RefCell`, `UnsafeCell`, `Ref` and `RefMut`.

While we had considered improving these names, no one was able to
produce a truly compelling alternative.

There is one substantive change here: the `get` method of `UnsafeSell`
is now marked `unsafe`. Merely getting a raw pointer to the contents is
not, by itself, an unsafe operation. (Consider that you can always
safely turn a reference into a raw pointer, and that raw pointer may
then be aliased by subsequent references.)

r? @alexcrichton