As consequence Buf::collect is removed as well, which is replaced with `Buf::into_bytes`. The advantage of `Buf::into_bytes` is that it can be optimized in cases where converting a `T: Buf` into a `Bytes` instance is efficient.

- Renames `bytes_vec` to `bytes_vectored` and `bytes_vec_mut` to
  `bytes_vectored_mut`.

- Removes `slice_to` and `slice_from`.

Shared references assert immutability, so any concurrent access would be UB
disregarding data race concerns.

There's no reason the user should be forced to wrap it in BufReader in
case the trait is needed, because the Reader has all the bits for
supporting it naturally.

The property the Buff and BuffMut can return shorter slice is quite an
important detail. Nevertheless, while it is mentioned in the
documentation, the wording makes it relatively easy to overlook. This
tries to bring more attention to it.

This lets us take Bytes and a &[u8] slice that is contained in it, and
create a new Bytes that corresponds to that subset slice.

Closes #198