code: public static IntStreamEx of(OptionalInt optional) {
          return optional.isPresent() ? of(optional.getAsInt()) : empty();
      }
docs: Returns a sequential {@code IntStreamEx} containing an
  {@link OptionalInt} value, if present, otherwise returns an empty
  {@code IntStreamEx}.
  
  @param optional the optional to create a stream of
  @return a stream with an {@code OptionalInt} value if present, otherwise
  an empty stream
  @since 0.1.1