move to GPLv2 versions of tree.hh and tree_util.hh

GPLv2 versions come from history of Cadabra
git clone http://repo.or.cz/r/cadabra.git
cd cadabra
git checkout 5c90c6700dc949a5c72240fe6c3fd14e68b6a062 src/tree.hh
git checkout b3bd02ebd6ae95cefd4aebade1f04283f009f4e3 src/tree_util.hh