Preserves original *.proto message and enum names

[Voronar]

Feature request (https://github.com/tokio-rs/prost/issues/207) implementation.

[LucioFranco]

If I understand correctly this change would allow prost-build to generate code with warnings?

[Voronar]

@LucioFranco Yes. But one can set warning disabling attribute like this:

pub mod protobuf {
  #![allow(non_camel_case_types)]
  include!(concat!(env!("OUT_DIR"), "/protoapi.rs"));
}