Fixes for implicit type vars and datatype decls in smt-lib

[Gbury]

Two fixes:

• do not create an instance of an implicit type variable when checking whether something is shadowed. Otherwise, that instance might be wrongly assumed to be used, and in the case of a datatype declaration that does not allow implicit type variables, this will raise a spurious error
• in datatype declarations, only explicit type variables are allowed, which was already correctly handled, but a shadowing warning was still emitted when one of the explicit type variables had the same name as one of the implicit one. In this case, there is no need for the warning as the implicit type variable cannot be used

This will close #253