Headless API requests

[dabreegster]

• [x] Make distance_crossed granular within a lane. (Also would help the trip timeline UI)
• [x] Expose total trip distance in /data/get-finished-trips
• [x] Edit a map in JSON and transform back to binary (attempted in https://github.com/dabreegster/abstreet/commit/6a2c391b8b717ef828b4efafe7d1645a6f7c0a9d)
• [x] a new API to expose a dependency graph of all agents waiting
• [ ] /sim/cancel-trip and/or /sim/change-trips. Only future trips can be changed. Have to validate endpoints. Maybe have to move a vehicle, if the next trip is cancelled.
• [x] /data/get-agent-positions should include vehicles that can't spawn at a border because stuff is in the way

[dabreegster]

No plans for further work here. A totally different headless API using protobufs, or maybe pyo3 / rextendr, may happen if there's demand