tail_head_slice is unsound

[gnzlbg]

The tail_head_slice method is unsafe, but impossible to use correctly because it returns a &[T] to uninitialized memory. It should return a &[MaybeUninit<T>].

cc @Shnatsel - while there is code using this method, I don't think there are currently any mis optimizations due to this difference (e.g. this is UB at the language spec level, but for the current toolchain, everything is fine).