Function map_editor::model::dump_to_osm

fn dump_to_osm(map: &RawMap) -> Result<(), Error>

Express a RawMap as a .osm file. Why not just save the RawMap? The format may change over time, and even if a RawMap is saved as JSON, manually updating it is annoying. This is used to create synthetic maps that will never go bad -- there will always be a pipeline to import a .osm file, so actually, .osm is a stable-over-time format.