mk: flatteno — nested list flattener

Three conde clauses: nullo tree -> nullo flat; pairo tree -> recurse on
car & cdr, appendo their flattenings; otherwise tree must be a ground
non-list atom (nafc nullo + nafc pairo) and flat = (list tree).

Works on ground inputs of arbitrary nesting:
  (run* q (flatteno (list 1 (list 2 3) (list (list 4) 5)) q))
    -> ((1 2 3 4 5))

7 tests, 253/253 cumulative. Phase 4 list relations now complete.

lib/minikanren/relations.sx

@@ -81,3 +81,18 @@
     (conde
       ((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
+    (tree flat)
+    (conde
+      ((nullo tree) (nullo flat))
+      ((pairo tree)
+        (fresh
+          (h t hf tf)
+          (conso h t tree)
+          (flatteno h hf)
+          (flatteno t tf)
+          (appendo hf tf flat)))
+      ((nafc (nullo tree)) (nafc (pairo tree)) (== flat (list tree))))))