topo: replace ConnectedComponents implementation with union-find

[Dan Kortschak]

We can do better with the performance of this function.

[Tobin C. Harding]

I copied path/disjoint.go to topo/. Modified it to store the node in the disjointSetNode struct. Then I implemented func (ds *disjointSet) connectedComponents() [][]graph.Node and used it to implement ConnectedComponents in topo.go. I ran benchmarks on the original implementation and the union-find implementation and the union-find was slower. Are you interested in seeing the code, or have you any further suggestions? No rush. See to it, if you choose to, at your leisure. Thanks.

[Dan Kortschak]

Closed after investigation in #190.