code: public <R> StreamEx<R> pairMap(BiFunction<? super T, ? super T, ? extends R> mapper) {
          PSOfRef<T, R> spliterator = new PairSpliterator.PSOfRef<>(mapper, spliterator());
          return new StreamEx<>(spliterator, context);
      }
docs: Returns a stream consisting of the results of applying the given function
  to the every adjacent pair of elements of this stream.
  
  <p>
  This is a <a href="package-summary.html#StreamOps">quasi-intermediate</a>
  operation.
  
  <p>
  The output stream will contain one element less than this stream. If this
  stream contains zero or one element the output stream will be empty.
  
  @param <R> The element type of the new stream
  @param mapper a non-interfering, stateless function to apply to each
  adjacent pair of this stream elements.
  @return the new stream
  @since 0.2.1