Function is_empty

src/fl/stl/unordered_map.h:813–815

    bool is_deleted(fl::size idx) const { return _deleted.test(idx); }

    bool is_empty(fl::size idx) const {
        return !is_occupied(idx) && !is_deleted(idx);
    }

    void mark_occupied(fl::size idx) {
        _occupied.set(idx);