feat: Implement subtypes

This should eventually be moved into typed-function itself, but for
  now it can be implemented on top of the existing typed-function.

  Uses subtypes to define (and error-check) gcd and lcm, which are only
  defined for integer arguments.

  Resolves #36.

src/generic/mod.mjs

@@ -0,0 +1,6 @@
+export const mod = {
+   'any,any': ({
+      subtract,
+      multiply,
+      quotient}) => (a,m) => subtract(a, multiply(m, quotient(a,m)))
+}