Parse .proto files

Parse .proto file definitions, not the protobuf text format serialization.

Files can be parsed using pure Rust parser (mod pure) or using the protoc command (mod protoc).

This crate is not meant to be used directly, but rather through the protobuf-codegen crate. If you think this crate might be useful to you, please consider creating an issue, until that this crate is considered to have no stable API.