mk: flat-mapo — concatMap-style relation

(flat-mapo rel l result): each element x of l is mapped to a list via
rel x list-from-x, and all such lists are concatenated to form result.

  (flat-mapo (fn (x r) (== r (list x x))) (list 1 2 3) q)
    -> ((1 1 2 2 3 3))

5 new tests, 515/515 cumulative.

lib/minikanren/relations.sx

@@ -72,6 +72,14 @@
       ((nullo l) (== result acc))
       ((fresh (a d new-acc) (conso a d l) (rel acc a new-acc) (foldl-o rel d new-acc result))))))
 
+(define
+  flat-mapo
+  (fn
+    (rel l result)
+    (conde
+      ((nullo l) (nullo result))
+      ((fresh (a d a-result rest-result) (conso a d l) (rel a a-result) (flat-mapo rel d rest-result) (appendo a-result rest-result result))))))
+
 (define
   membero
   (fn
@@ -80,6 +88,7 @@
       ((fresh (d) (conso x d l)))
       ((fresh (a d) (conso a d l) (membero x d))))))
 
+
 (define
   not-membero
   (fn
@@ -88,7 +97,6 @@
       ((nullo l))
       ((fresh (a d) (conso a d l) (nafc (== a x)) (not-membero x d))))))
 
-
 (define
   subseto
   (fn