std: deprecate cast::transmute_mut.

Turning a `&T` into an `&mut T` is undefined behaviour, and needs to be
done very very carefully. Providing a convenience function for exactly
this task is a bad idea, just tempting people into doing the wrong
thing.

(The right thing is to use types like `Cell`, `RefCell` or `Unsafe`.)

cc https://github.com/mozilla/rust/issues/13933