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/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(''), [])
+   })
+})