[CN-Test-Gen] Purely unfolding implementation

[ZippeyKeys12]

Add an implementation of test generation.

This version generates Google Test harnesses with RapidCheck generators, one per path through the precondition.
It unfolds recursive predicates up to the specified max depth.

I still have to add some tests for CI.

[dc-mak]

Looking reasonable, some minor suggestions. I know some steps for the CI are a pain to add but a few things in CI will make everyone's development process smoother.

[dc-mak]

I'm approving, though I would like to see the changes requested in the follow-up PR (CI, subcommands, rename car to carrier, and a more explanatory file explainer).

[dc-mak]

Ah damnit the squashed commit message didn't include "This version generates Google Test harnesses with RapidCheck generators, one per path through the precondition. It unfolds recursive predicates up to the specified max depth." which would have been a better summary than what I wrote. Oh well. Next time I'll just ask you to squash it instead.