Three conde clauses: nullo tree -> nullo flat; pairo tree -> recurse on
car & cdr, appendo their flattenings; otherwise tree must be a ground
non-list atom (nafc nullo + nafc pairo) and flat = (list tree).
Works on ground inputs of arbitrary nesting:
(run* q (flatteno (list 1 (list 2 3) (list (list 4) 5)) q))
-> ((1 2 3 4 5))
7 tests, 253/253 cumulative. Phase 4 list relations now complete.
