libextra: Another round of de-`Cell`-ing.

34 uses of `Cell` remain.
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.
compiletest: Remove uses of `Cell`.
auto merge of #10791 : pcwalton/rust/decelling, r=alexcrichton

34 uses of `Cell` remain.

r? @alexcrichton