Implement 'stack of routers' for MVC

This is the MVC companion to https://github.com/aspnet/Routing/pull/122

As routing flows, routes replace the route data and mutate a copy. This
allows users to make changes that dirty the data without affecting
undesired state changes.
btw I like `RouteDataTest` because the next work item here is to make sure that all the same kinds of stuff work with `DataTokens`, so those will go here too.