mk: pairlisto — relational zip

(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.

lib/minikanren/relations.sx

@@ -190,6 +190,14 @@
       ((== n :z) (== result x))
       ((fresh (n-1 mid) (== n (list :s n-1)) (rel x mid) (iterate-no rel n-1 mid result))))))
 
+(define
+  pairlisto
+  (fn
+    (l1 l2 pairs)
+    (conde
+      ((nullo l1) (nullo l2) (nullo pairs))
+      ((fresh (a1 d1 a2 d2 d-pairs) (conso a1 d1 l1) (conso a2 d2 l2) (conso (list a1 a2) d-pairs pairs) (pairlisto d1 d2 d-pairs))))))
+
 (define
   everyo
   (fn