Testing All The Differentials

The frule and rrule testing functionality is now pretty decent. However, there is not good utility functionality to assist in writing tests for how each of the various AbstractDifferentials play with each of the frules and rrules.

Ideally frule_test and rrule_test will be set up such that each possible combination of arguments to each pushforward and pullback are tested separately, and for the most part automatically.

edit: on second thoughts, we should consider whether or not we actually want to test all of the possible combinations, or just make sure that each argument sees each AbstractDifferential, since the pushforwards and pullbacks are linear in their arguments.

Start by reading the existing frule_test and rrule_test functionality, then inspect how pushforward and pullback receive AbstractDifferentials. Resolve whether coverage should enumerate argument combinations or ensure each argument sees each differential. Done means automatic tests cover the agreed scope for the differentials, frules, and rrules.