Cleanup modality transfers

Preserving notes:

- let bike trips start/end anywhere, not just halfway down road
- codewise
- precompute everything, store directly in map
- detect connectivity breaks or changes in this while editing
- be clear which can fail and not
- simplest start: just lane connectivity
- fancier: exact positions to do transfers
- things needed
- driving <-> parking lanes (0, 1, or 2 parking)
- driving <-> offstreet building parking
- bldg (sidewalk) <-> where to start/end a bike trip
- bldg (sidewalk) <-> parking lane
- bldg (sidewalk) <-> driving lane where we should start searching for parking
- existing code to reconsider
- find_driving_lane_near_building
- is children_forwards and children_backwards useful?
- figure out how to model lack of sidewalks
- oh yeah, parking without sidewalks