code: public <R> StreamEx<R> mapKeyValue(BiFunction<? super K, ? super V, ? extends R> mapper) {
          return this.<R> map(toFunction(mapper));
      }
docs: Returns a {@link StreamEx} consisting of the results of applying the
  given function to the keys and values of this stream.
  
  <p>
  This is an <a href="package-summary.html#StreamOps">intermediate</a>
  operation.
  
  @param <R> The element type of the new stream
  @param mapper a <a
  href="package-summary.html#NonInterference">non-interfering </a>,
  <a href="package-summary.html#Statelessness">stateless</a> function to apply to key and
  value of each {@link Entry} in this stream
  @return the new stream