Fix: shortcut for provers which are unknown

Johannes, Yannick: yes this is what you think (good news)
but this is not yet completed (bad news)

(we do not have to use the ghost construct here, but it looks nicer)

But:
 - problem with BuiltIn, Bool, Tuple, Unit and theory
 - work only with "why" and "whyml" files format

warning for possibly useless quantifiers (unless a leading underscore is used)