Merge: Make all Sets cloneable

Since there is no reason to forbid it (or I have not seen one at the very least), here's a simple `clone` method that should work on any `Set`

Pull-Request: #1893
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>