Entry API for HashSet

[ajtribick]

It would in some cases be useful to have an equivalent of the entry API available for HashSet. The case I ran into was wanting to remove values from two sets only if both sets contain the values. With the current API, this would be something like:

if set1.contains(&value1) && set2.remove(&value2) {
    set1.remove(&value1);
}

which does two lookups in set1 in the case that the values should be removed: as far as I can see there is no way to avoid doing the duplicate lookup. A similar situation would arise in the case where values should only be inserted if neither set contains the values.

It is possible to emulate this with a HashMap<T, ()>, e.g.

if let Entry::Occupied(entry) = map1.entry(value1) {
    if map2.remove(&value2).is_some() {
        entry.remove();
    }
}

but would be a nice-to-have for HashSet as well.