XPath: Use node pointers in step_push/step_fill

Using pointers instead of node/attribute objects allows us to use knowledge
about the tree to guarantee that pointers are not null. This results in
less null checks (10-20% speedup with optimizations enabled) and less
function calls (5x speedup with optimizations disabled).

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1072 99668b35-9821-0410-8761-19e4c4f06640