Stop signs not generated at intersection of one-way streets

https://github.com/a-b-street/abstreet/blob/a75f874dbf2cdb4aaad3d33127d7ad332d8af44e/map_model/src/objects/stop_signs.rs#L96-99

The comment references 'degenerate roads and deadends', but since only incoming roads are inserted into ss.roads, a simple intersection of two one-way streets does not result in an all-way stop as expected but instead no stop signs. The obvious change to `map.get_i(id).roads.len() <= 2` results in stop signs being added where a two-way street of any rank has an outward T-intersection, because the following logic "If all roads have the same rank, all-way stop. Otherwise, everything stops except the highest-priority roads." only counts incoming roads.