2htdp/universe: adjust universe `loop` to make it a loop

Neither the body nor handler functions in `with-handlers` are
in tail position with respect to `with-handlers`.
2htdp/universe: adjust world `loop` to make it a loop

Similar to the server-side problem, but on the client side. In a
game where the server drives the clients with frequent messages
through `on-tick`, per-message growth in the continuation can
matter a lot.