XPath: Introduce _first/_any set evaluation modes

Sometimes when evaluating the node set we don't need the entire set and
only need the first element in docorder or any element. In the absence of
iterator support we can still use this information to short-circuit
traversals.

This does not have any effect on straightforward node collection queries,
but frequently improves performance of complex queries with predicates
etc. XMark benchmark gets 15x faster with some queries enjoying 100x
speedup on 10 Mb dataset due to a significant complexity improvement.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1067 99668b35-9821-0410-8761-19e4c4f06640

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