fix MaybeADT ctor Some: constructed value *must not* do role Definition; use copies when converting to P6Terms; refactor EtaReduction tests
remove P6 impl of eta-contract; refactor EtaReduction tests
add test confirming that findFP returns the *1st* arg to predicate rather than the *2nd*, as soon as predicate returns #true
finish EtaReduction with pure lambda repr: implement etaReduce via findFP, in lambda