Add two bucket

[ageron]

That was a fun exercise!

For the solution, I wrote a general-purpose BFS implementation. The advantage is that it can be reused in other exercises (e.g., it would have been handy for the connect exercise). The main downsides are:

• the bfs function outputs the shortest path even though all we really need is the length of the shortest path
• as a result, the solution is a bit longer than it could be
• and the type annotations of the bfs function could potentially be confusing to beginners

I don't think these downsides matter much, given that the .meta/Example.roc file is not shown to the end user, it's mostly there to ensure that the tests work well. Moreover, this exercise has difficulty 6, so it's not really meant for complete beginners, we can assume they understand type annotations by now.

Wdyt?

[isaacvando]

Nice!! I think this is a great opportunity to show off a general BFS.