Typing

I was thinking about adding `typed/racket` support to this library. There's a lot of ways to do this. Here's some ways that look simple enough to me.

- We could make a new dir within `rackcheck-lib` called `typed` and re-export everything from `rackcheck`.
- The same, but within a new lib `rackcheck-typed`

Alternatively (or even in parallel?), one could gradually type the library and get it to work that way.

What do you think?