mk: not-membero — relational "x is not in l"

Mirrors the structure all-distincto already uses internally: walk the
list, ensure each element is not equal to x via nafc, recurse on tail.
Useful as a constraint-style filter:

  (membero x (list 1 2 3 4 5))
  (not-membero x (list 2 4))
    -> x in {1, 3, 5}

4 new tests, 382/382 cumulative.

lib/minikanren/relations.sx

@@ -50,6 +50,14 @@
       ((fresh (d) (conso x d l)))
       ((fresh (a d) (conso a d l) (membero x d))))))
 
+(define
+  not-membero
+  (fn
+    (x l)
+    (conde
+      ((nullo l))
+      ((fresh (a d) (conso a d l) (nafc (== a x)) (not-membero x d))))))
+
 (define
   reverseo
   (fn
@@ -82,6 +90,7 @@
       ((nullo l) (nullo p))
       ((fresh (a d perm-d) (conso a d l) (permuteo d perm-d) (inserto a perm-d p))))))
 
+
 (define
   flatteno
   (fn
@@ -97,7 +106,6 @@
           (appendo hf tf flat)))
       ((nafc (nullo tree)) (nafc (pairo tree)) (== flat (list tree))))))
 
-
 (define
   rembero
   (fn