Removing the use of leveled tactics wit_tacticn. It is now handled
through a unique generic argument, and the level is only considered
at parsing time.

This may introduce unnecessary parentheses in Ltac printing though,
as every tactic argument is collapsed at the lowest level. I assume
this does not matter that much, and anyway Ltac printing is quite
bugged as of today.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16616 85f007b7-540e-0410-9357-904b9bb8a0f7