Added a generic notion of hook. Hooks are functions to be set
exactly once at runtime, often to reduce the mutual dependency of
modules. This module permits to track them more easily.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16509 85f007b7-540e-0410-9357-904b9bb8a0f7
Use the Hook module here and there.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16510 85f007b7-540e-0410-9357-904b9bb8a0f7