Implement non-recursive node copying
Implement non-recursive node copying

This makes node copying 6% faster, prevents it from ever running out of
stack space and makes the profiling results more actionable for profilers
that can't merge information from recursive calls.

git-svn-id: 3715ede575e2934be0fecc9022d600f6f7ea101b@1027 99668b35-9821-0410-8761-19e4c4f06640

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml