scan_xxx method could panic
when buckets don't exist Bucket::Empty(), just Bucket::Removed() and Bucket::Contains(k,v) if we exec scan_xxx method, the key doesn't match every existed bucket then the scan_xxx method will trigger panic.
when buckets don't exist Bucket::Empty(), just Bucket::Removed() and Bucket::Contains(k,v) if we exec scan_xxx method, the key doesn't match every existed bucket then the scan_xxx method will trigger panic.