- 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

With this if foo is a mutable slice, it is possible to do

foo.into_buf().put_u32_le(42);

Before this patch into_buf would create a Cursor<&'a [u8]> and it
would not be possible to write into it.