Better `set_if_unequal()`

[kpreid]

Cell::set_if_unequal() has the caveat that "this executes PartialEq::eq() with the lock held". We can do better than that:

1. Acquire the lock.
2. Swap the new value in.
3. Release the lock.
4. Compare the old value.
5. If unequal, send a notification.

This is a visible behavior change because it swaps values even if unnecessary, so if downstream cares at all about differences ignored by PartialEq like pointer addresses, it could detect the difference, but that should be acceptable in nearly all cases. Still, I think the new operation should be a new method with clear documentation.

[kpreid]

Oops, I missed a thought: we need access to the new value in order to do the comparison afterward. This means cloning it — another way in which this proposed different version isn't a pure win. It will still be reducing the work done with the lock held, but cloning with the lock held is another routine O(size_of_value) activity so I'm not sure whether it's worth making this complication.