Add protobuf-style variable-length serialization / deserialization

[arnohaase]

Variable-length integer encoding (protobuf style) is present in many network and file protocols. While it is not at the same level of 'standard' as LE / BE encoding, I would find it a highly useful addition to the Buf / BufMut traits.

I'm happy to provide a PR, but I wanted to check if you feel this fits into the crate's scope.

[Darksonn]

My gut feeling is that this is outside of the scope.

[arnohaase]

So - providing a separate crate extending Buf / BufMut functionality would probably be the best way to go?

[Darksonn]

Sure, you can definitely do that.

[arnohaase]

In case you are interested: https://github.com/arnohaase/bytes-varint