Transit shortest route

A journey planner stores a transit map as `links`, where each entry is a string like "Bank>Oval" naming one track segment between two stations. Trains run both ways along a segment, so the order inside an entry carries no meaning. A rider boarding at `start` wants to reach `finish` riding as few segments as possible, and each segment ridden counts as one hop. Return the smallest number of hops, 0 when `start` and `finish` name the same station, and -1 when no route exists, which includes the case where a station never appears on the map. The map may list the same segment twice, may list a segment from a station back to itself, and may contain loops.