crate, in preparation for future focus on travel demand models that
incorporate more per-person info.
Note: I'm also a bit tempted to try to further split sim into a
"high-level" layer that orchestrates spawning and different legs of a
trip, from the "low-level" layer that moves pedestrians and vehicles.
That could _possibly_ pave the way for someday using a different traffic
simulation backend with more realistic movement mechanics.
Step 1: just get the synthpop crate to build
