Add the PureXML.XPointer namespace resolving the shorthand bare-name form
(id), the element() scheme (child-element positions from an id or the
document root), and the xpointer() scheme (a full XPath expression). Scheme
parts chain with the XPointer fallback rule: the first non-empty selection
wins. element() data is translated to the equivalent XPath and evaluated by
PureXML.XPath, so no separate evaluator is needed.
Closes#46Closes#47Closes#23
