More granual `build.rs` companion: `links.rs`

Potentially wild idea — a new buildscript directive that tells Cargo "everything is ready to build the crate" that allows the crate to be built in parallel to the rest of the build script, which is only allowed to emit directives (and built objects) that affect linking and/or Cargo's rebuild heuristics after that point. This would allow any calls to other build systems to not block the pipeline, hopefully improving parallel availability instead of bottlenecking on compiling external code that may not even be needed for the current (e.g. check) build.