Add sound implementation of Bytes for &[u8]

[Thomasdezeeuw]

See rust-lang/rfcs#2930 (comment) and rust-lang/rfcs#2930 (comment).

Example:

let mut buf = [0u8];
let maybe = unsafe { &mut *((&mut buf) as *mut [u8] as *mut [MaybeUninit<u8>]) };
// After this line `buf` will contain uninitialized value, which is UB
maybe[0] = MaybeUninit::uninit();

[Thomasdezeeuw]

This should also revert all the changes in the tests/docs from 4f8731c.

[Thomasdezeeuw]

Changing to issue to track adding back an implementation of Bytes for &[u8].