refactor: Switch to 'map-like object keyed by string and type vector' format

See https://code.studioinfinity.org/glen/nanomath/wiki/Item-Specifications.
  Also stubs out the TypeDispatcher, mocking the merge function, so we
  can see that all of the proper things will be added.

  Ready for initial implementation of the TypeDispatcher.

src/number/helpers.js

@@ -0,0 +1,5 @@
+import {Returns, onType} from '../core/helpers.js'
+import {Number} from './Number.js'
+
+export const plain = f => onType(
+   Array(f.length).fill(Number), Returns(Number, f))