streets2osm

Preserving Ben's idea from https://github.com/a-b-street/abstreet/pull/897:

> One interesting mode of operation would be to output RawMap as OSM xml in some "normal form": consistent tagging, all geometry running down the center of the road width, intersection:areas (or whatever they are called) for intersections, etc. without worrying about the connectivity. I would be interested to see what it looks like using those outputs directly in existing tools like [Map Machine](https://github.com/enzet/map-machine). It might be a good way to integrate with such tools.