Support "resources" in `listening`.

[izaakschroeder]

So a lot of times you'd like to have a resource constructed when the server starts listening, use it during the request lifecycle, and then dispose of it when the server shuts down. Being able to return new apps in listening may be able to allow this.

listening(() => {
  const resource = new Thing();
  return compose(
    assign({resource}),
    close(() => resource.dispose()),
  );
});