Organize crates

We have lots of them now, and the flattened directory listing is a little overwhelming? Here's a vague attempt at organizing, not happy with it yet.

apps
- fifteen_min
- game
- ltn
- map_editor
- osm_viewer
- parking_mapper
- santa

project / logistic
- cloud
- release
- tests
- traffic_seitan
- web

importing
- importer
- collisions
- convert_osm
- kml
- traffic_signal_data
- popdat

ui
- map_gui
- piggyback
- widgetry
- widgetry_demo

uncategorized
- abstio
- abstutil
- cli
- geom
- headless
- map_model
- sim
- updater