add set difference, intersection, union, fix sum error op

[tgolsson]

This adds HashSet::{difference, intersection, union} methods. To make that doable it adds more blanket implementations for TypeOf so we can take self as Ref<Self> in receiver position. This required removing one static type declaration. This required removing one static type declaration for `Ref - I can't really tell the extent of the impact that has, but that specific incantation seems to only be used in one FromValue declaration.

Also fixes a minor copy-paste error in Iterator summation leading to very confusing error messages.

[udoprog]

Maybe clear self.other once iteration is completed to release the reference?

[udoprog]

Looks good! Minor nonblocking nit.

[tgolsson]

@udoprog this should do if I understood you correctly.

[udoprog]

Yeah, exactly! I think the loop can be cleaned up a bit:

let other = self.other.take()?;

loop {
    let item = self.this.next()?;

    if other.set.contains(&item) {
        self.other = Some(other);
        return Some(item);
    }
}

I.e. if other is not set, the iterator has completed. If this completes, then other is not restored.