(pairlisto l1 l2 pairs): pairs is the zipped list of pairs (l1[i] l2[i]).
Recurses on both l1 and l2 in lockstep, building pairs in parallel.
Runs forward, can recover l1 given l2 and pairs, can recover l2 given
l1 and pairs. Different-length lists fail.
5 new tests, 431/431 cumulative.
