Add hedgehog generator for SOP

[pieter-bos]

I recently wrote this hedgehog generator for SOP as an experiment to see if it's feasible to write interesting properties about the functions in natnormalise. I didn't get farther than writing the obvious property for simplifySOP, which already results in some counterexamples.

For now I've opened this draft to sollicit a bit of feedback:

• Do you think it's a good idea to include property-based tests to natnormalise (and maybe extra in the future)?
• If so, how should we organize resolving the resulting counterexamples? Should we just make a big PR with all the fixes, or include test fixtures first?

Thanks!