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/number/isZero.mjs

@@ -0,0 +1,3 @@
+export * from './Types/number.mjs'
+
+export const isZero = {number: () => n => n === 0}