added bytes::Buf and bytes::BufMut support to Ring

[carlsverre]

I would love to have native Buf and BufMut support for easier integration between vmap::Ring and prost. You can find prost here: https://github.com/tokio-rs/prost

Documentation on traits:

• Buf
• BufMut

Happy to move the code around, add tests, or change features. Not sure how you'd like these kinds of extensions to work.

And thanks for such an awesome crate!

[kalamay]

Hey @carlsverre, thanks so much for the PR! Sorry I've been unable to get some eyes on this, but I'll take a look.