Allow access to bytes::Bytes from protobuf::Chars

[SamHDev]

For my current task, I want to slice the underlying bytes of protobuf::Chars from a regex capture group. This seems impossible with the current API.

If this was to be implemented an AsRef<_> would likely be the best way to implement this.

I'mma fork and add this small snippet, lemme know what you think?

- Sam.