Removing the dependency of every level of tactic ATSs on glob_tactic_expr.
Instead of putting the body directly in the AST, we register it in a table.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@17077 85f007b7-540e-0410-9357-904b9bb8a0f7