Avoid defensive copying in keys protocols

[daxfohl]

Measurement and control keys protocols both do defensive copies before returning. However they are also used in hot loops where perf is important. It's likely better to specify that dunders should return return a frozenset instead, especially since most ops will return an empty one, which is a singleton requiring no allocation.

This may be impossible to do in a completely non breaking way, however I can't think of a reason users would rely on these returning mutable sets.

Update: The mypy types of the return values is AbstractSet, not Set. And FrozenSet is also a subclass of AbstractSet so this is not breaking.