Fix `route_instance_path`

`route_instance_path` was not working with nested resources (the one that use `belongs_to`). It's now fixed.

`route_instance_path` was returning a symbol like `:admin_post_path` while `route_collection_path` was returning the path like `/admin/posts`. `route_instance_path` now returns the path.

I also refactored the route generation into `RouteBuilder`.

:cake: