Use petgraph traits whereever possible

[sebschmi]

petgraph already defines many traits for unidirected and undirected graphs, we should use them instead of spending time on creating our own.

[sebschmi]

This would require substantial work on petgraph, as the crate is not developed trait-based, but rather implementation based.
On the other hand, it would allow us to use some of their basic algorithms for SCCs and WCCs.