Allocation in Intersection::seek

[PSeitz]

We should remove the allocation in Intersection::seek

    fn seek(&mut self, target: DocId) -> DocId {
        self.left.seek(target);
        let mut docsets: Vec<&mut dyn DocSet> = vec![&mut self.left, &mut self.right];
        for docset in &mut self.others {
            docsets.push(docset);
        }
        let doc = go_to_first_doc(&mut docsets[..]);
        debug_assert!(docsets.iter().all(|docset| docset.doc() == doc));
        debug_assert!(doc >= target);
        doc
    }