graph: add node, edge and line iterators

Created by: kortschak

I have largely left the changes in the client code as mechanical, using the graph package's helper, except where there's an obvious simplification or improvement in performance. The changes here should have reasonably limited negative impact on performance and will allow improvments in performance in some cases (when reflect gets map iterator support we'll see a benefit, and I'll write an unsafe equivalent of that API).

Please take a look.

This should go in after #565 since I'll update this to add the necessary changes in that code, so please review that first.

Fixes gonum/graph#18. Fixes gonum/graph#46. Updates #570.