.intersection(), .union() etc methods in trait std::container::Set use
internal iters. Remove these methods from the trait.

I reported issue #8154 for the reinstatement of iterator-based set algebra
methods to the Set trait.

For bitv and treemap, that lack Iterator implementations of set
operations, preserve them as methods directly on the types themselves.

For HashSet, these methods are replaced by the present .union_iter()
etc.