mk: samelengtho — equal-length relation

Recurses positionally, dropping a head from each list each step. Both
arguments can be unbound, giving the natural enumeration:

  (run 3 q (fresh (l1 l2) (samelengtho l1 l2) (== q (list l1 l2))))
    -> (((), ())                         empty/empty
        ((_.0), (_.1))                   pair of 1-element lists
        ((_.0 _.1), (_.2 _.3)))          pair of 2-element lists

5 new tests, 293/293 cumulative.

lib/minikanren/relations.sx

@@ -124,3 +124,11 @@
     (conde
       ((== n :z) (fresh (d) (conso elem d l)))
       ((fresh (n-1 a d) (== n (list :s n-1)) (conso a d l) (nth-o n-1 d elem))))))
+
+(define
+  samelengtho
+  (fn
+    (l1 l2)
+    (conde
+      ((nullo l1) (nullo l2))
+      ((fresh (a d a-prime d-prime) (conso a d l1) (conso a-prime d-prime l2) (samelengtho d d-prime))))))