Support for Marshalling to Proto Objects

[kevinkreiser]

hi! would you be willing to support a slight refactor to marshal not just to bytes but rather to the precursor mvt spec proto objects? this is useful for grpc services which will automatically do the marshalling to bytes as part of their request handling. ive not tested this but wanted to do the PR as an illustration. let me know if its something you'd be ok supporting or not and i can write tests and clean it up if so!