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/bigint/_all.mjs

@@ -52,4 +52,16 @@ describe('bigint', () => {
       assert.deepStrictEqual(bo.sqrt(-3249n), bo.complex(0n, 57n))
    })
 
+   it('computes gcd', () => {
+      assert.strictEqual(math.gcd(105n, 70n), 35n)
+   })
+
+   it('computes lcm', () => {
+      assert.strictEqual(math.lcm(105n, 70n), 210n)
+      assert.strictEqual(math.lcm(15n, 60n), 60n)
+      assert.strictEqual(math.lcm(0n, 17n), 0n)
+      assert.strictEqual(math.lcm(20n, 0n), 0n)
+      assert.strictEqual(math.lcm(0n, 0n), 0n)
+   })
+
 })

test/complex/_all.mjs

@@ -29,4 +29,10 @@ describe('complex', () => {
          math.complex(ms.negate(ms.sqrt(0.5)), ms.sqrt(0.5)))
    })
 
+   it('computes gcd', () => {
+      assert.deepStrictEqual(
+         math.gcd(math.complex(53n, 56n), math.complex(47n, -13n)),
+         math.complex(4n, 5n))
+   })
+
 })

test/custom.mjs

@@ -39,6 +39,7 @@ describe('A custom instance', () => {
       assert.strictEqual(pm.subtract(5, 10), -5)
       pm.install(complexAdd)
       pm.install(complexNegate)
+      pm.install(complexComplex)
       // Should be enough to allow complex subtraction, as subtract is generic:
       assert.deepStrictEqual(
          pm.subtract({re:5, im:0}, {re:10, im:1}), {re:-5, im: -1})

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)
+   })
 })