Expose ColliderSet::{take_modified, take_removed}

These are practically necessary when working without a PhysicsPipeline. A larger change could avoid exposing `ModifiedObjects`, but it seems like it might be useful as well. Alternatively, `BroadPhaseBvh::update` could be modified to pull this data from the `ColliderSet` itself. In that case the public interface could be narrowed to a method on `ColliderSet` to clear the modified/removed sets, but this would make it impossible for a user to inject their own modified/removed handles. I'm not sure if that's useful.