Refactor to use shared Graph code

[dabreegster]

This cuts over to share https://github.com/a-b-street/15m/tree/main/graph with the 15m tool, towards the first use case of a-b-street/15m#17.

• Much less code here, and the abstractions over in the common Graph crate fit nicely here
• Routes shown here have precise positions along roads now
• I spotted some cases around UTD where an individual route looked more correct with the changes
• Score mode looks a bit different before/after, but really need to do Generate better desire lines #22 before polishing results here more

This unlocks doing #18 quite easily as a next step