This enables optional route parameters.
Required some code changes in the RouteCollection class; once we
actually use optional route parameters, we will have to see whether
route generation for those works as expected.
Closesflarum/core#108
All routes are now stored in a RouteCollection, which is then used
for dispatching by the (reusable) RouterMiddleware.
This change also entails moving all routes to the service providers.
This may be changed again later, and is done for convenience reasons
right now.
This will be able to dispatch PSR-7-style requests to any callback
that returns a proper response object.
Largely based on my original work for FluxBB 2.0.
