mk: tako + dropo — Peano-indexed prefix and suffix

(tako n l prefix) — prefix is the first n elements of l.
(dropo n l suffix) — suffix is l after dropping the first n.

Both use a Peano natural for the count. Round-trip holds:
  (tako n l) ⊕ (dropo n l) = l   (verified by an end-to-end test)

9 new tests, 368/368 cumulative.

lib/minikanren/relations.sx

@@ -172,3 +172,19 @@
     (conde
       ((fresh (x) (conso x (list) l) (== init (list))))
       ((fresh (a d d-init) (conso a d l) (conso a d-init init) (init-o d d-init))))))
+
+(define
+  tako
+  (fn
+    (n l prefix)
+    (conde
+      ((== n :z) (== prefix (list)))
+      ((fresh (n-1 a d p-rest) (== n (list :s n-1)) (conso a d l) (conso a p-rest prefix) (tako n-1 d p-rest))))))
+
+(define
+  dropo
+  (fn
+    (n l suffix)
+    (conde
+      ((== n :z) (== suffix l))
+      ((fresh (n-1 a d) (== n (list :s n-1)) (conso a d l) (dropo n-1 d suffix))))))