Move LittleLock to using RAII

This moves the locking/waiting methods to returning an RAII struct instead of
relying on closures. Additionally, this changes the methods to all take
'&mut self' to discourage recursive locking. The new method to block is to call
`wait` on the returned RAII structure instead of calling it on the lock itself
(this enforces that the lock is held).

At the same time, this improves the Mutex interface a bit by allowing
destruction of non-initialized members and by allowing construction of an empty
mutex (nothing initialized inside).