feat(returnTypeOf): Support template and upper bound types

Also changed the notation for an upper bound template to the more
  readable 'T:number' (instead of 'T (= number', which was supposed to
  look like the non-strict subset relation).

test/_pocomath.mjs

@@ -23,6 +23,17 @@ 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')
+      math.negate(math.complex(1.2, 2.8)) // TODO: make this call unnecessary
+      assert.strictEqual(
+         math.returnTypeOf('negate', 'Complex<number>'), 'Complex<number>')
+      assert.strictEqual(math.returnTypeOf('add', 'number,number'), 'number')
+      assert.strictEqual(math.returnTypeOf('add', 'NumInt,NumInt'), 'NumInt')
+      assert.strictEqual(math.returnTypeOf('add', 'NumInt,number'), 'number')
+      assert.strictEqual(math.returnTypeOf('add', 'number,NumInt'), 'number')
+      assert.deepStrictEqual(  // TODO: ditto
+         math.add(3, math.complex(2.5, 1)), math.complex(5.5, 1))
+      assert.strictEqual(
+         math.returnTypeOf('add', 'Complex<number>,NumInt'), 'Complex<number>')
    })
 
    it('can subtract numbers', () => {

test/core/_utils.mjs

@@ -0,0 +1,8 @@
+import assert from 'assert'
+import * as utils from '../../src/core/utils.mjs'
+
+describe('typeListOfSignature', () => {
+   it('returns an empty list for the empty signature', () => {
+      assert.deepStrictEqual(utils.typeListOfSignature(''), [])
+   })
+})