grpc: improve examples

1. The grpc examples share a cargo package with tonic's examples. Tonic's examples require protoc in your PATH which we'd like to avoid as a strict requirement for grpc. We should move the grpc examples to `grpc/examples`, or move the tonic examples to `tonic/examples`.

2. The grpc examples currently use a `build.rs` method that requires rebuilding every time, because it outputs the files into `src` and cargo considers them dirty even though they were produced deterministically during the build process. Instead we should output them into the OUT_DIR, and commit them into another (non-src/...) directory and copy them into OUT_DIR from there when not regenerating from them with protoc (so the examples are trivially runnable).