Replaces the grpclib dependency in betterproto with Twirp. This codegens Twirp client and servers that obey the wire protocol spec (and also match the Twirp python lib's template)

If we really wanted to do this correctly, we could do any or all of the following:

• Write some tests :)
• Write our own implementation of the twirp's wire protocol - this would allow us to remove the google protobuf dependency, since betterproto doesn't depend on it either
• Instead of ripping out grpc, make it an option at compilation / pip install, so that we could submit this patch back upstream and not have to maintain a fork