When doing setoid_rewrite through rewrite, do resolution of classes
in w_unify_to_subterm, deactivated by previous patch (fixes RelationAlgebra).


git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16579 85f007b7-540e-0410-9357-904b9bb8a0f7