One more fix for rewrite: disallow resolving of the (partial) constraints
happening silently in w_unify and handle this explicitely. Class resolution
filters now can test the existential key. Fixes Ergo contrib.


git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16571 85f007b7-540e-0410-9357-904b9bb8a0f7