mk: iterate-no — apply a relation n times

(iterate-no rel n x result) holds when applying the 2-arg relation rel
n times (Peano n) starting from x produces result. Base case: zero
iterations means result equals x. Recursive case: rel x mid, then
iterate-no n-1 from mid.

Generalises common chains:
  succ iteration:  (iterate-no succ-rel n :z q) -> n in Peano
  list growth:     (iterate-no cons-rel n () q) -> n-element list

4 new tests, 426/426 cumulative.

lib/minikanren/relations.sx

@@ -182,6 +182,14 @@
       ((nullo l1) (nullo l2))
       ((fresh (a d a-prime d-prime) (conso a d l1) (conso a-prime d-prime l2) (rel a a-prime) (mapo rel d d-prime))))))
 
+(define
+  iterate-no
+  (fn
+    (rel n x result)
+    (conde
+      ((== n :z) (== result x))
+      ((fresh (n-1 mid) (== n (list :s n-1)) (rel x mid) (iterate-no rel n-1 mid result))))))
+
 (define
   everyo
   (fn