provide route collection hierarchy

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Here is an implementation : https://github.com/prestaconcept/PrestaCMSCoreBundle/blob/route-collection-hierarchy/Model/RouteManager.php#L36

My goal is to provide a sitemap.xml. I think it could be great to add this to the cmf route API.