mk: foldr-o — relational right fold

Takes a 3-arg combiner relation, a list, and an initial accumulator,
produces the right-fold result. (rel a tail-result result) combines
the head with the recursive result.

Examples:
  (foldr-o appendo (list (list 1 2) (list 3) (list 4 5)) (list) q)
    -> ((1 2 3 4 5))                ; flatten

  (foldr-o conso (list 1 2 3) (list) q)
    -> ((1 2 3))                    ; rebuild list

4 new tests, 505/505 cumulative.

lib/minikanren/tests/foldr-o.sx

@@ -0,0 +1,38 @@
+;; lib/minikanren/tests/foldr-o.sx — relational right fold.
+
+(mk-test
+  "foldr-o-empty"
+  (run* q (foldr-o conso (list) (list 99) q))
+  (list (list 99)))
+
+(mk-test
+  "foldr-o-conso-rebuilds-list"
+  (run* q (foldr-o conso (list 1 2 3) (list) q))
+  (list (list 1 2 3)))
+
+(mk-test
+  "foldr-o-appendo-flattens"
+  (run*
+    q
+    (foldr-o
+      appendo
+      (list
+        (list 1 2)
+        (list 3)
+        (list 4 5))
+      (list)
+      q))
+  (list (list 1 2 3 4 5)))
+
+(mk-test
+  "foldr-o-with-acc-init"
+  (run*
+    q
+    (foldr-o
+      conso
+      (list 1 2)
+      (list 9 9)
+      q))
+  (list (list 1 2 9 9)))
+
+(mk-tests-run!)