graph/multi: clean up missed changes and bugs

Created by: kortschak

This fixes some bugs in the multigraph handling of edges from lines and finishes changes that I had intended to go into the iterator PR (the change in container for graph.*Lines in the multi.*Edge types).

It has also raised an issue of how I have been dealing with empty collections. With the old approach we could always return a nil and people could still iterate over it without needing to check for nil. This is no longer true. We currently don't specify the nil-ness of returns for collections, so this is not terrible, but it might be an idea to tighten that down. I'm raising this here since you can see the impacts with these changes, but I will file an issue (#614 (closed)).

Please take a look.