[WIP] add compiler pass for bundles to register mappings

use the compiler pass added to the doctrine bridge to provide a nice compiler pass for bundles. See https://github.com/symfony/symfony/pull/7599

i will also write documentation once we agree on the exact format. for now see https://github.com/FriendsOfSymfony/FOSUserBundle/pull/1081 for an example how this is intended to be used.