Simplify the str-to-decimal function

pact/contracts/util-strings.pact

@@ -226,11 +226,10 @@
     (let* ((is-negative (= "-" (take 1 in)))
            (in (if is-negative (drop 1 in) in))
            (parts (split "." in)))
-      (enforce (or? (= 1) (= 2) (length parts)) "Invalid format")
-      (let* ((int-part (at 0 parts))
-             (has-decimal (= 2 (length parts)))
-             (dec-part (if has-decimal (at 1 parts) "0"))
-             (precision (if has-decimal (length dec-part) 0))
+      (enforce (or? (is-singleton) (is-pair) parts) "Invalid format")
+      (let* ((int-part (first parts))
+             (dec-part (if (is-pair parts) (last parts) "0"))
+             (precision (if (is-pair parts) (length dec-part) 0))
              (dec-multiplier (^ 0.1 (dec precision)))
              (str-to-dint (lambda (x) (dec (str-to-int 10 x))))
              (val (+ (str-to-dint int-part) (* dec-multiplier (str-to-dint dec-part)))))