Rebuild the XPath engine on the parent-aware tree. Add an XPath node
abstraction (tree, attribute, namespace) with document order, and navigate
all thirteen axes: child, descendant, parent, ancestor, following-sibling,
preceding-sibling, following, preceding, attribute, namespace, self,
descendant-or-self, ancestor-or-self. Support the ./../@/// abbreviations
and processing-instruction() node tests. Reverse axes number nearest-first
for positional predicates; results return de-duplicated in document order.
The attribute axis excludes xmlns declarations; the namespace axis surfaces
in-scope namespace nodes.
Closes#40
