mk: lengtho-i — integer-indexed length

Drop-in fast replacement for Peano lengtho when the count fits in a
host integer. Two conde clauses: empty list -> 0; recurse, n = 1 +
length(tail). Uses pluso-i so the length walks to a native int.

5 new tests, 486/486 cumulative.

lib/minikanren/intarith.sx

@@ -109,3 +109,11 @@
     (conde
       ((nullo l) (== total 1))
       ((fresh (a d rest-prod) (conso a d l) (producto d rest-prod) (*o-i a rest-prod total))))))
+
+(define
+  lengtho-i
+  (fn
+    (l n)
+    (conde
+      ((nullo l) (== n 0))
+      ((fresh (a d n-1) (conso a d l) (lengtho-i d n-1) (pluso-i 1 n-1 n))))))