`futures_task::waker_ref()` is unsound

The requirements of `futures_task::waker(a)` and `futures_task::waker_ref(&a)` for all `a` should be the same. However in the current implementation, the former requires `a` to be `'static`, while the latter does not. This enables a static `Waker` to be obtained from a non-static `a`, resulting in some potential use-after-free problem.