other set operations

[Geal]

we already have union and intersection for sets, it would make sense to have the difference operation too. And maybe add and remove too (they can be implemented using union and difference, but it would make them easier to use with dedicated calls)

[divarvel]

If we relax constraints on sets following #135, especially if we allow nesting sets, we won't be able to keep the overloaded methods on sets (which would have been a convenient way to provide add / remove with overloaded union / difference)