Also apply generalization to inductive type constructors

* src/elab.ml (generalize): Change calling convention.
(infer_and_generalize_type, infer_and_generalize_def): Adjust accordingly.
(lexp_parse_inductive.make_args): Generalize constructors.
(var_of_ovar): Move to lexp.ml.

* src/opslexp.ml (check', get_type): Fix scoping of `level` in Inductive.

* btl/builtins.typer (Eq_comm): Take advantage of improvements in
gneralization and unification.

* btl/pervasive.typer (BoolMod): Avoid `typecons`'s generalizing.
(LList): New type, to test typecons's generalization.

* src/lexp.ml (var_of_ovar): Move from elab.ml.
(lexp_unparse): Simplify.

* src/pexp.ml (pexp_u_ind_arg): Remove.