`HsDataDefn` allows newtypes with multiple constructors.
Now, it is possible to construct a HsDataDefn with dd_ND = NewType but (dd_cons = _:_:_ or dd_cons = []), which is wrong, since a newtype must have exactly 1 constructor.
Now, it is possible to construct a HsDataDefn with dd_ND = NewType but (dd_cons = _:_:_ or dd_cons = []), which is wrong, since a newtype must have exactly 1 constructor.