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.
