Make IndexSet::intersection() work with stdlib's hashset

[Manishearth]

There's no reason it shouldn't work with the stdlib hashset. Perhaps we can add a second method for this?

[cuviper]

It could work with lots of container types -- anything that has a contains method. But really, intersection amounts to just set.iter().filter(|item| other.contains(item)).

[bluss]

We don't inadvertently want to be responsible for a traitification of set collections, I guess? It's an awkward question.