The main entry point to highnoon. An App can be launched as a server
or mounted into another App.
Each App has a chain of Filters
which are applied to each request.
A response to be returned to the client.
You do not always need to use this struct directly as endpoints can
return anything implementing Responder. However this is the most flexible
way to construct a reply, and it implements Responder (the “identity” implementation).