Add some optional glue code to the bytes crate

[fogti]

It would be nice if this crate would provide optional glue code to the bytes crate, with which I mean implementing the bytes::{Buf,BufMut} traits, to allow easier integration with other existing code, e.g. poll-buf-utils. It is probably a good idea to support a wide range of bytes crate versions, as the Buf & BufMut traits are present in that crate since almost the first versions, and haven't changed at all.