Restrict unnecessarily broad import
Add Plotkin's CBV CPS transformation
Separate generation of fresh names from fresh vars

Generating fresh variables is just a convenience method, and now with
CBPV we also need to generate fresh names.
Rename method

We need 'freshName' for the newly introduced method. It's not clear to
me whether we need both ones, but I think yes.
Fix thinko in Plotkin's CPS
Comment out aborted code

This code tries to represent CBPV terms as a subclass of normal terms.
That's a hack without a use case yet, but it's not clear to me how to
proceed after processing CBPV --- maybe we'll need such a hack, maybe
we'll just erase CBPV into lambda-calculus (and/or A-normal form).
Sketch term-to-CBPV conversion