Add latest protobuf definitions, rework protoc generation, ver bump

- Instead of modifying things in src/ (which is actually verboten for
  good reason on places like crates.io), we now keep everything
  generated by the build script in OUT_DIR, and include the generated
  file.
- (This required some hacks to work around Rust issue #18810.)
- We're now using the latest protobuf defs from the sigalor repo,
  and we're on the latest version.