librustuv: RAII-ify `Local::borrow`, and remove some 12 Cells.
libstd: Remove some cells involved in
`deschedule_running_task_and_then`.
librustpkg: Make `io::ignore_io_error()` use RAII; remove a few more
cells.
libstd: Remove `Cell`s that were used because of `finally` by converting
their `finally` blocks to RAII.