First, we change the Protobuf definition includes to use an exact path relative to our crate's directory, instead of the OUT_DIR. This fixes build when a combination of stable and nightly toolchains is used (nightly is needed for the fuzzing targets).
Another change is a slight fix in the panic handler conditional compilation. Fuzzing is using the crate with `features = ["test"]`, but doesn't turn on the `test` cfg.
[no changelog]
