Now, idtac closures use maps instead of association list.

The semantics changed slightly so it may break some scripts, though
it is very unlikely, as they would have to be quite intricated and
poorly written. Indeed, the test-suite passed just fine.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16604 85f007b7-540e-0410-9357-904b9bb8a0f7