IntDisjointSets: make indexing safer

This should fix #45.

Note that I had to remove the `push!(s, 17)` method, because there is no way to support such things in a safe manner. If there are not 17 elements in `s.parents`, it yields nonsense anyway.

I have also back-ported this to 0.2, which I'll submit separately once we get this into acceptable shape.