librustdoc: Don't use `finally`, shaving off a `Cell`.
librustdoc: Remove a couple of `Cell`s.
libextra: Remove various cells involved in `Arc`s.

I could have done this by making `Arc` use RAII, but this is too
involved for now.
librustuv: Change `with_local_io` to use RAII.
libstd: Change `atomically` to use RAII.
libstd: Remove two uses of `Cell`.
libstd: Remove `Cell` from the library.