(md5_init): Use memcpy for initializing the state vector.

(COMPRESS): New macro, wrapping _nettle_md5_compress.
(md5_update): Use MD_UPDATE.
(md5_digest): Inline md5_final processing. Use MD_PAD and
_nettle_write_le32.
(md5_final): Deleted.

Rev: nettle/md5.c:1.3