:arrows_counterclockwise: :sparkles: clean clean-n-arena

At the Factual Clojure meetup on the ninth (it's fun; you should go!),
@ztellman pointed out that (obviously) there's no reason for
clean-n-arena to be a macro when ordinary, mundane recursion is fine.

Also, given that the browser game works well and has more features,
let's not bother to maintain the terminal demo.