Custom structure for disjoint trees

[gdalle]

We need something better than DataStructures.DisjointSets to iteratively build a set of disjoint trees, keeping track of the BFS order

[amontoison]

#190 will help because we will have our own structure Forest. We can probably update it to keep track of the BFS order.

[amontoison]

I think we can't do better than our current structure Forest.