mk: foldl-o — relational left fold

Complement to foldr-o. The combiner relation has signature
(acc head new-acc) — accumulator first.

Examples:
  (foldl-o pluso-i (list 1 2 3 4 5) 0 q)  -> (15)
  (foldl-o *o-i    (list 1 2 3 4)   1 q)  -> (24)
  (foldl-o (fn (acc x r) (conso x acc r))  ; flipped conso
           (list 1 2 3 4) (list) q)        -> ((4 3 2 1))   ; reverse

5 new tests, 510/510 cumulative.

lib/minikanren/tests/foldl-o.sx

@@ -0,0 +1,48 @@
+;; lib/minikanren/tests/foldl-o.sx — relational left fold.
+
+(mk-test
+  "foldl-o-empty"
+  (run* q (foldl-o pluso-i (list) 42 q))
+  (list 42))
+
+(mk-test
+  "foldl-o-sum"
+  (run*
+    q
+    (foldl-o
+      pluso-i
+      (list 1 2 3 4 5)
+      0
+      q))
+  (list 15))
+
+(mk-test
+  "foldl-o-product"
+  (run*
+    q
+    (foldl-o
+      *o-i
+      (list 1 2 3 4)
+      1
+      q))
+  (list 24))
+
+(mk-test
+  "foldl-o-reverse-via-flip-conso"
+  (run*
+    q
+    (foldl-o
+      (fn (acc x r) (conso x acc r))
+      (list 1 2 3 4)
+      (list)
+      q))
+  (list (list 4 3 2 1)))
+
+(mk-test
+  "foldl-o-with-init"
+  (run*
+    q
+    (foldl-o pluso-i (list 1 2 3) 100 q))
+  (list 106))
+
+(mk-tests-run!)