feat(PocomathInstance): Add subtypesOf and isSubtypeOf methods

Note this involves refeactoring the internal subtype tracker to keep
  subtypes in a list sorted in topological order.

test/_pocomath.mjs

@@ -22,6 +22,7 @@ describe('The default full pocomath instance "math"', () => {
 
    it('can determine the return types of operations', () => {
       assert.strictEqual(math.returnTypeOf('negate', 'number'), 'number')
+      assert.strictEqual(math.returnTypeOf('negate', 'NumInt'), 'NumInt')
    })
 
    it('can subtract numbers', () => {