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/relations.sx

@@ -64,6 +64,14 @@
       ((nullo l) (== result acc))
       ((fresh (a d r-rest) (conso a d l) (foldr-o rel d acc r-rest) (rel a r-rest result))))))
 
+(define
+  foldl-o
+  (fn
+    (rel l acc result)
+    (conde
+      ((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
   membero
   (fn
@@ -80,6 +88,7 @@
       ((nullo l))
       ((fresh (a d) (conso a d l) (nafc (== a x)) (not-membero x d))))))
 
+
 (define
   subseto
   (fn
@@ -88,7 +97,6 @@
       ((nullo l1))
       ((fresh (a d) (conso a d l1) (membero a l2) (subseto d l2))))))
 
-
 (define
   reverseo
   (fn