Stream.iterate() should allow covariant seed and UnaryOperator args

Stream.iterate() currently has two overloads:

    static <T> Stream<T> iterate(T seed, Predicate<? super T> hasNext, UnaryOperator<T> next)
    static <T> Stream<T> iterate(T seed, UnaryOperator<T> f)

This prevents certain combinations of existing values and unary operators from being used. For example:

    UnaryOperator<String> op = s -> s;
    Stream<CharSequence> scs = iterate("", op); // error

If the methods' declarations were changed like so:

    static <T, S extends T> Stream<T> iterate(S seed, Predicate<? super S> hasNext, UnaryOperator<S> next)
    static <T, S extends T> Stream<T> iterate(S seed, UnaryOperator<S> f)

Then the above call to iterate() would no longer result in an error.