Consider replacing petgraph with a simpler data structure

[bhearsum]

Petgraph was chosen as the data structure for Genome networks for two reasons:

1. It has a lot of helpers when it comes to finding upstream and downstream nodes, connecting edges, etc.
2. It let me sidestep any possible ownership issues with implementing a linked list manually

However, as the implementation got more complicated it felt like a strong possibility that petgraph may be hindering performance, overkill, or otherwise not the right tool for the job.

Before proceeding any further here, it would be ideal to some profiling to see if it is (or isn't) a hot spot.

If it is, looking for an alternative data structure libraries or biting the bullet and implementing the connections manually (f it's even feasible) may be worth considering.