START, END, CODING-SYSTEM and DESTINATION.
START and END are buffer positions.
-Optional 4th arguments DESTINATION specifies where the encoded text goes.
-If nil, the region between START and END is replace by the encoded text.
+Optional 4th argument DESTINATION specifies where the encoded text goes.
+If nil, the region between START and END is replaced by the encoded text.
If buffer, the encoded text is inserted in that buffer after point (point
does not move).
In those cases, the length of the encoded text is returned.