Introduce std::mutable with Mut<T>, a mutable slot
extra::rc: Replace RcMut<T> with Rc<Mut<T>>
Simplify Cell
Wip: Update to use Mutable instead of with_mut_ref

Update region infer

Update to Mut
Port rt/uv use of Cell::new_empty and *Cell<T>
Remove Cell::new_empty and Cell::put_back
Replace Cell with Mut