- ng_refiner:

- ng_refiner:
  now the expected type is `XTNone (was None), `XTSome (was Some, also
  was `Term), `XTSort (any sort, was `Sort), `XTInd (any (co)inductive type).
  `XTProd (was `Prod) is used in "try_coercions"
- we replaced some instances of None with `XTSort or `XTInd through the code.
  lib, lambda, and lambdadelta compile,
  in case of errors like "the term ... is not a sort" or
  "the term ... is not a (co)inductive type", revert to `XTNone in the
  corresponding check.
- Now generated objects have the `Generated attribute properly set
- lambda: many "?" removed because of the improvement in the refiner
          other instances of "?" removed for other reasons