Introduce std::mutable with Mut<T>, a mutable slot
extra::rc: Replace RcMut<T> with Rc<Mut<T>>
Port users of Cell::with_mut_ref to use Mut<T>
Port users of Cell::put_back to Mut<T>
Replace Cell with Mut

Replace the common Cell idiom with Mut<T>

before:

	let upvar_cell = Cell::new(x);
	do spawn {
		let xyz = upvar_cell.take().recv()
	}

after:

	let upvar_cell = Mut::new_some(x)
	do spawn {
		let xyz = upvar_cell.take_unwrap().recv()
	}