DOM Level 3 Core: key() redundant if XPath supported

The use cases for Node.key are probably covered if an implementation supports the DOM 3 XPath.  The primary use that I saw was to maintain a Map that associated additional data with particular Nodes, if the generate-id() method could be easily invoked on arbitrary nodes (and returned invariant responses despite document mutation which would need to be asserted in the DOM 3 XPath spec), then it would be simple to maintain an external map with data associated with Nodes.