Cleanup: Move CBPV into its own files

We shouldn't intrude in the namespace of all users of
Parametric.Syntax.{Type,Term}.

While creating CBPVTerm, I also fixed the comment for Vals and Comps.

I also had to add the modularization boilerplate (forwarding structures
and such) for both files.

View with `-C30` to view actual changes.
Prototype proof that static caching is type-correct

See comments for extra info.