add documentation, move the implementation to a private folder and add the csexp? predicate

Hello! I'm currently working on a decentralized game development environment based on Racket, and found your spritely project to be a perfect match!

I discovered by the same time canonical s-expressions, and found your implementation seems to be the only one in the Racket ecosystem. Except it cannot be found when using the Racket search bar, because it has no documentation!

So I wrote some, and also added the csexp? predicate.

Let me now if you see any issue with what I did :)

PS: I've been following this project on Mastodon for some time now, and I'd be glad to provide some of my time and help this project grow!