mk: phase 4C — permuteo (with inserto helper)

inserto a l p: p is l with a inserted at some position. Recursive: head
of l first, then push past head and recurse.

permuteo l p: classical recursive permutation. Empty -> empty; otherwise
take a head off l, recursively permute the tail, insert head at any
position in the recursive result.

7 new tests including all-6-perms-of-3 as a set check (independent of
generation order). 208/208 cumulative.

lib/minikanren/relations.sx

@@ -65,3 +65,19 @@
     (conde
       ((nullo l) (== n :z))
       ((fresh (a d n-1) (conso a d l) (== n (list :s n-1)) (lengtho d n-1))))))
+
+(define
+  inserto
+  (fn
+    (a l p)
+    (conde
+      ((conso a l p))
+      ((fresh (h t pt) (conso h t l) (conso h pt p) (inserto a t pt))))))
+
+(define
+  permuteo
+  (fn
+    (l p)
+    (conde
+      ((nullo l) (nullo p))
+      ((fresh (a d perm-d) (conso a d l) (permuteo d perm-d) (inserto a perm-d p))))))