Cache Rules in AbstractInterpreter

[willtebbutt]

If a rule has already been derived, don't re-derived it, just look it up from a cache of rules.

[yebai]

Good one -- it's strange that some tests fail with cached rules.

[willtebbutt]

There's quite a lot of state associated to the object returned by build_rrule!!, so the test failures mean that something isn't composing properly there at the minute. 🤞 it's something simple.