Proof-of-concept: moved four easy-to-handle generic arguments to
their own file, Stdarg.

This required a little trick to correctly handle wit_* naming. We
use a dynamic table to remember exactly where those arguments come
from.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16587 85f007b7-540e-0410-9357-904b9bb8a0f7