Rewrite RcDom's serialize implementation to be iterative

If we use a work queue for child nodes that still need to be serialized like Servo's [SerializationIterator](https://github.com/servo/servo/blob/b4a2d60b25ced61a7f5b23df33036b8d55ee0ce8/components/script/dom/servoparser/html.rs#L152-L262), this will be more efficient and have better performance than a recursive implementation.