Refactor accessing node type into a macro

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1042 99668b35-9821-0410-8761-19e4c4f06640
Optimize node_copy_tree by switching to pointers

xml_node objects carry an overhead since they perform NULL checks - in
case of copying a hierarchy we know that we only traverse valid nodes so
we don't need to do this. This makes copyless copy 16% faster.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1043 99668b35-9821-0410-8761-19e4c4f06640