change structure & add laws for enums

[jdegoes]

Review by @garyb.

[garyb]

Thanks, this module has needed some love since the start. It was only thrown in for DateTime originally so was never properly considered (and that module needs work as well now I think of it).

I guess it will be a little more effort to define an Enum instance now, but I think it will be worth it.

[paf31]

Looks good, thanks.

[paf31]

This reminds me of something I had meant to bring up: I think it's good to start documenting things like this in comments, so that we can use docgen to render nice docs when it's ready. But what do you think of using markdown in comments to structure the documentation?

[garyb]

Sounds perfect to me, I already tend to write comments in that style.