Implement structuregraphs using rustworkx or graphtool

[kjappelbaum]

What inconvenient in the current implementation

1. Slow. For large graphs, networkx is painfully slow. If one were to reimplement structure graphs I would reconsider this dependency.
2. Double graph layer. At least in my use I quite often find myself working on graph.graph. Perhaps there should only be one ground truth for nodes and edges
3. There is a link to the structure but it is not really clear what can mutate what (in fact, I think immutability would be much clearer)

What is convenient in the current implementation

There is a link to the structure.