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.

test/number/_all.mjs

@@ -27,4 +27,7 @@ describe('number', () => {
       assert.deepStrictEqual(no.sqrt(-16), no.complex(0,4))
    })
 
+   it('computes gcd', () => {
+      assert.strictEqual(math.gcd(15, 35), 5)
+   })
 })