fix: RuntimeError: dictionary changed size during iteration

[YLivay]

Fixes an error that happens when Marshal registers a new instance while Marshal.get_rule is executing in a separate thread.

Fixes: #545

[vchudnov-g]

Hi @YLivay ! Thanks for proposing this change. Some general comments:

• Could you add tests for this, ideally not just to test the functionailty works as intended, but also that it gets around the problem in RuntimeError: dictionary changed size during iteration #545?
• What is the performance hit of _cls.instances,copy(). For typical cases, is it greater or less than the RW lock you propose as an alternative in RuntimeError: dictionary changed size during iteration #545? How does it scale?

[vchudnov-g]

Also, I suggest rebasing to HEAD--- that should get the pre-submits to pass (or alert us to an issue!). Thanks!