Making the refcounting atomic isn't that hard (there are people who use an unofficial atomic ARC mode) but the cycle collector is quite tricky to multi-thread.
I'm more interested though in ensuring that plain old locking works with `ref` types. It's quite unexplored territory but it makes sense: You leave out the atomic instructions for a single refcount field but you protect it and other RCs at the same time with a plain lock. Single threaded case remains fast and the multi-threaded case has a higher chance of actually being correct.
