public <R> Stream<R> map(BiFunction<? super K, ? super V, ? extends R> mapper) {
        requireNonNull(mapper);
        return map(e -> mapper.apply(e.getKey(), e.getValue()));
    }
Returns a stream consisting of the results of applying the given
function to the elements of this stream.
<p>
This is an intermediate operation.

@param <R>     the element type of the new stream
@param mapper  a non-interfering, stateless function to apply to each 
               key-value pair
@return        the new stream
@throws NullPointerException if any of the provided parameters are {@code null}
common-parent/mapstream/src/main/java/com/speedment/common/mapstream/MapStream.java