mk: counto — count occurrences of x in l (intarith)

Walks the list with a recursive count. On a head match, recurse and
add 1 via pluso-i; on no match (nafc), recurse forwarding the count.
Empty list yields 0.

6 new tests, 532/532 cumulative.

lib/minikanren/intarith.sx

@@ -127,3 +127,11 @@
       ((fresh (a d r-rest start-prime) (conso a d l) (conso (list start a) r-rest result) (pluso-i 1 start start-prime) (enumerate-from-i start-prime d r-rest))))))
 
 (define enumerate-i (fn (l result) (enumerate-from-i 0 l result)))
+
+(define
+  counto
+  (fn
+    (x l n)
+    (conde
+      ((nullo l) (== n 0))
+      ((fresh (a d n-rest) (conso a d l) (conde ((== a x) (counto x d n-rest) (pluso-i 1 n-rest n)) ((nafc (== a x)) (counto x d n))))))))