Thread-safe subclass mode

[adetaylor]

Right now, if C++ passes our subclass unique_ptr to a different thread, all bets are off. We should provide a configuration option to make a thread safe version using Mutex and Arc instead of RefCell and Rc. Maybe we can use archery.

[tychedelia]

Unfortunately, archery does not support weak pointers, which is a significant roadblock. They also implement their own smart pointer types, which would require us to leak these to users instead of Arc and Rc. Since it also doesn't help much with abstracting over Mutex and RefCell, I think another approach is needed, unfortunately.