eliminate build dependency on installed protoc, re-export prost::Message

[dvdsk]

Uses protobuf-src to build statically linked protoc from scratch. This removes the requirement to have protoc installed (which quickly gets annoying when building for N different platforms).

To decode a model into ModelProto you need to have the prost::Message trait in scope. If you depend on a different version of prost then candle decoding will fail. By re-exporting prost users can depend directly on our version.