Use petgraph crate for Graph operations

[XVilka]

Would make development easier since many operations are already implemented out of the box

• https://github.com/petgraph/petgraph
• https://docs.rs/petgraph
• https://crates.io/crates/petgraph

[endeav0r]

I like our current graph implementation, and don’t see a strong reason to change it :(.

I’ll leave this open for a few weeks for counter-points, but I think Falcon’s graph implementation is here to stay.