code: public <V extends Comparable<? super V>> IntStreamEx sortedBy(IntFunction<V> keyExtractor) {
          return sorted(Comparator.comparing(keyExtractor::apply));
      }
docs: Returns a stream consisting of the elements of this stream, sorted
  according to the natural order of the keys extracted by provided
  function.
  
  <p>
  For ordered streams, the sort is stable. For unordered streams, no
  stability guarantees are made.
  
  <p>
  This is a <a href="package-summary.html#StreamOps">stateful intermediate
  operation</a>.
  
  @param <V> the type of the {@code Comparable} sort key
  @param keyExtractor a
  <a href="package-summary.html#NonInterference">non-interfering
  </a>, <a href="package-summary.html#Statelessness">stateless</a>
  function to be used to extract sorting keys
  @return the new stream