Proto Files

• sentencepiece_model.proto is downloaded from the C++ original source, in https://github.com/google/sentencepiece/, but it should match the one used by the github.com/eliben/go-sentencepiece library.

Because of protoc unique file naming requirement (!?), described in email thread in https://groups.google.com/g/protobuf/c/UWWuoRWz1Uk, we compile by first creating a unique prefix directory. See gen_protos.sh script.