[iOS] Displaying view controller on parent RIB in nested hierarchy

[alexvbush]

Original issue: uber/RIBs#302

I have this hierarchy:

I'm trying to display view controller of component D (low-level) in view controller of component Root (which has navigation controller, so basically I need to call show(vc, sender: nil)).

However in this nested hierarchy I have no access to view controller of component Root in children components. How can I achieve that?