Clojure.spec tests

[jjl]

I really like clojure.spec. It would be pretty awesome if we could find a way to use it to provide clearer error messages if you misconfigure things, especially since with the one-way casts we're likely to add quite a few more keys.

We can't actually release it with spec in yet because it requires an alpha clojure 1.9, but when 1.9 is out, I think we should seriously consider requiring it, not least for the generative testing.

Thoughts? @ShaneKilkelly

[JuneKelly]

No objections, once Clojure 1.9 is out :)

EDIT: more than just 'no objections', seems like a good idea, especially if it improves error messages.