Do not allocate huge temporary memory areas and objects while encoding

Do not allocate huge temporary memory areas and objects while encoding
for file I/O, thus reducing an enormous memory usage for large buffers.
See http://lists.gnu.org/archive/html/emacs-devel/2013-10/msg00180.html.
* coding.h (struct coding_system): New member raw_destination.
* coding.c (setup_coding_system): Initialize it to zero.
(encode_coding_object): If raw_destination is set, do not create
dst_object.  Add comment.
* fileio.c (toplevel): New constant E_WRITE_MAX.
(e_write): Do not encode more than E_WRITE_MAX characters per one loop
iteration.  Use raw_destination if E_WRITE_MAX characters is encoded.