Avoid allocation when `Take` or `Chain` is composed of `Bytes`
objects.
This works now for `Take`.
`Chain` it works if the requested bytes does not cross boundary
between `Chain` members.
The `bytes()` / `bytes_mut()` name implies the method returns the full
set of bytes represented by `Buf`/`BufMut`. To rectify this, the methods
are renamed to `chunk()` and `chunk_mut()` to reflect the partial nature
of the returned byte slice.
`bytes_vectored()` is renamed `chunks_vectored()`.
Closes#447
