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.