refactor: Simpler merging mechanism

Merging of Pocomath modules is eased by allowing one PocomathInstance to
  be merged into another. That allows types, for example, to be exported
  as a PocomathInstance (so there is no need for a special identifier
  convention for types; they can be directly added with an installType
  method). Also, larger modules can just be exported as an instance, since
  there is more flexibility and more checking in merging PocomathInstances
  than raw modules.

src/generic/Types/generic.mjs

@@ -1,2 +1,5 @@
-export const Type_undefined = {test: u => u === undefined}
+import PocomathInstance from '../../core/PocomathInstance.mjs'
+const Undefined = new PocomathInstance('Undefined')
+Undefined.installType('undefined', {test: u => u === undefined})
+export {Undefined}