This PR adds implementation for generating type signatures for those generated by protoc --ruby_out=... and grpc_tools_ruby_protoc.

The code generation part is almost complete under the assumption that the appropriate counterpart exists in the signatures for google-protobuf and grpc, namely:

• google-protobuf
  • class Google::Protobuf::AbstractMessage
  • class Google::Protobuf::RepeatedField[out Elem, in ElemWrite]
  • class Google::Protobuf::Map[out Key, out Value, in KeyWrite, in ValueWrite]
  • class Google::Protobuf::EnumDescriptor
  • Those generated from google.protobuf.* packages
• grpc
  • module GRPC::GenericService
  • class GRPC::ActiveCall::SingleReqView
  • class GRPC::ActiveCall::MultiReqView[out Req]
  • class GRPC::ActiveCall::Operation[out Result]
  • class GRPC::ClientStub
  • class GRPC::Core::Call
  • class GRPC::Core::CallCredentials