Rewrite theory

[TOTBWF]

Description

This PR defines various closures of relations, and also adds the basics of rewriting theory.

Highlights include:

• Newmans Lemma
• Commutative Union Lemma
• Church-Rosser is equivalent to confluence

Notes

I'm not totally sure about the organization here; Data.Rel does seem to be the best home for properties about relations, but I'm not 100% sold on how I've organized rewrite theory.

[TOTBWF]

I have a better idea of how to do this now!