- Keep the refinement of existing evars comming from unification with a rewrite lemma.
- Do an nf_meta before get_type_of in unify_to_type to avoid a possible NotAnArity exception,
raised by type_of_*_knowing_parameters. 


git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16592 85f007b7-540e-0410-9357-904b9bb8a0f7