I'd really like to avoid keeping these all down here. We already have leaks above from the early returns.

Can you add the relevant bits of [libextobjc](https://github.com/jspahrsummers/libextobjc.git) as a subtree (based on the instructions for [objc-build-scripts](https://github.com/jspahrsummers/objc-build-scripts?source=c)) so we can use `@onExit`?