Eliminate XPathNode

XPathNode requires :position, but in most case, this :position is not used at all, and just doing useless packing/unpacking.

Author: tompng

lib/rexml/functions.rb

@@ -53,7 +53,7 @@ def Functions::last( )
     end
 
     def Functions::position( )
-      @@context[:index]
+      @@context[:position]
     end
 
     # Returns the size of the given list of nodes.