code: public static <T> Collector<T, ?, OptionalLong> maxIndex(Comparator<? super T> comparator) {
          return minIndex(comparator.reversed());
      }
docs: Returns a {@code Collector} which finds the index of the maximal stream
  element according to the specified {@link Comparator}. If there are
  several maximal elements, the index of the first one is returned.
  
  @param <T> the type of the input elements
  @param comparator a {@code Comparator} to compare the elements
  @return a {@code Collector} which finds the index of the maximal element.
  @see #maxIndex()
  @since 0.3.5