Add law functions

[hdgarrood]

WIP. This will fix #110 when it's done.

I'd appreciate a quick review now just to check that we agree on this style before I go ahead and do all the others in the same way.

[garyb]

Nice 👍

[hdgarrood]

Come to think of it, I wonder if a better formulation would be returning a { lhs :: a, rhs :: a } for the appropriate type a? That way, you're free to use some other form of equality checking. E.g. if both sides evaluate to functions of some kind, you might also generate a number of inputs and check those functions agree for all the inputs. Or if a was Number, you could use an approximate equality function.

[hdgarrood]

I'm going to close this for now since it's not had any progress in almost two years now (!)