Find the route

You will write a route finder for a few reactions. It looks at every place you can reach in one step, then in two steps, and so on, until it reaches the goal. This is a breadth-first search.

The idea. Each compound class is a place. Each reaction is an arrow, and it adds some carbons to the chain. The shortest route is the one with the fewest arrows.

Your tasks.

1. Complete next_places(place) so that it returns the list of places that one reaction reaches.
2. Complete steps(start, goal) so that it returns the least number of reactions.
3. Complete carbons_added(start, goal) so that it returns the carbons added along the shortest route.