Smaller files: don't serialize turn geometry?

Not sure this will work, but it's a good little exploratory project. map_model/src/turn.rs stores geom: PolyLine. Last time I used the serialized_size_bytes stuff in map.rs, these polylines constituted a huge part of the binary map files. The release size is slowly growing, so it'd be nice to trim this down.

Maybe we could skip serializing the PolyLine, then in Map::new after deserializing, go fill it in again. Should measure the impact on file sizes and initial map loading time.