be more stricte in accepting inductives and algebraics
1. We only accept an algebraic type declaration T 'a1 ... 'aN whenever every constructor has ls_value = Some (T 'a1 ... 'aN), no type variable renaming is allowed. 2. We only accept an inductive predicate declaration P (_x1 : T1) ... (_xN : TN) whenever every inductive clause has a conclusion of the form (P (t1 : T1) ... (tN : TN)), no type variable renaming is allowed. 3. To this purpose, we must typecheck the whole (mutual) inductive declaration group in the same denv. This must be ok, since user-named type variables cannot be destructively instantiated anyway. However, I'd like Jean-Christoph to check my changes in Typing.add_inductives.