Change BufMut methods that expose maybe-uninitialized bytes (#305)

- The return type of `BufMut::bytes_mut` is now
  `&mut [MaybeUninit<u8>]`.
- The argument type of `BufMut::bytes_vectored_mut` is now
  `&mut [bytes::buf::IoSliceMut]`.
- `bytes::buf::IoSliceMut` is a `repr(transparent)` wrapper around an
  `std::io::IoSliceMut`, but does not expose the inner bytes with a safe
  API, since they might be uninitialized.
- `BufMut::bytesMut` and `BufMut::bytes_vectored_mut` are no longer
  `unsafe fn`, since the types encapsulate the unsafety instead.