Codebase refactor ideas

[dabreegster]

Low priority ideas to improve things.

• Stop manually generating GJ features within "application logic". Write type-safe structs and use geojson::ser annotations on geometry, then in the wasm lib.rs layer, do the actual serializing. Ideally this could also hook into https://github.com/Aleph-Alpha/ts-rs or similar to generate wasm.ts on the other end, rather than manually writing it.
• Try replacing lots of network representation and routing logic with https://github.com/a-b-street/15m/tree/main/graph. That upstream library would have to be generalized for modal filters and turn restrictions. It's certainly going to be faster to initially iterate on those things directly in this codebase.