Supporting tests for mutating rules

I know mutating rules aren't fully supported yet, but I've started implementing some, following the guidelines in this tweet. To support testing mutating functions, I think only a few changes are needed:

1. Because the mutating function might use the values of an input before overwriting it, it is only safe to call the function multiple times as the testers do on copies of the inputs. Specifically deep copies. So the idea is to only pass deep copies of inputs to the functions in the testers.
2. Because the pullback must undo the mutation, after calling the pullback, check that the copies of the inputs that were passed to the rrule are approximately equal to the original inputs.

For an example of how this might work, see this working implementation.

Am I missing anything, or are there any objections to me preemptively making these changes?

Start by comparing the existing testers with the working example in test/diffrules.jl. Confirm whether testers pass deep copies to mutating functions and verify after pullback that the copied inputs approximately match their originals; done means these checks support mutating-rule tests without breaking existing behavior.