Module spawner

Intermediate structures used to instantiate a Scenario. Badly needs simplification: https://github.com/a-b-street/abstreet/issues/258

Structs

StartTripArgs 🔒

We need to remember a few things from scenario instantiation that’re used for starting the trip.

Enums

TripSpec 🔒

Functions

driving_goal 🔒

end_sidewalk_spot 🔒

start_sidewalk_spot 🔒