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/complex/abs.mjs

@@ -1,5 +1,5 @@
 export * from './Types/Complex.mjs'
 
-export const abs = {Complex: ({sqrt, add, multiply}) => z => {
-    return sqrt(add(multiply(z.re, z.re), multiply(z.im, z.im)))
-}}
+export const abs = {
+    Complex: ({sqrt, 'absquare(Complex)': absq}) => z => sqrt(absq(z))
+}