Optimize XPath step

If a predicate of xpath is position-independent, we don't need to create nodesets that has many duplicated nodes with different positions.
Implements optimization of preceding-sibling/following-sibling with simple positional predicates as an example.
We can add more optimizations for other axis in the future.

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.