Fix O(N) performance of DOMNode::replaceChild() and DOMNode::removeChild()

Don't loop over all children to determine if the target node really is a
child, just trust the parent pointer.

Fix O(N) performance of DOMNode::replaceChild() and DOMNode::removeChild()

Don't loop over all children to determine if the target node really is a
child, just trust the parent pointer. Add tests.

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