Drop tonic dependency

[declark1]

This PR makes changes required to drop tonic as a dependency from mocktail. It is still used as a client in mocktail-tests.

Changes:

• Add status.rs
  • Move StatusCode here
  • Add Code, based on tonic::Code (with http code mapping changes)
• Drop CodeExt
• Update gRPC service:
  • Use BoxBody (with hyper::Error) instead of tonic::body::BoxBody and associated empty() body helper instead of tonic::body::empty_body()
  • Use hyper::Error in Result instead of tonic::Status
  • Use Code for grpc-status header instead of tonic::Code

Closes #36
Closes #43