(COMPRESS): New macro wrapping _nettle_sha256_compress.

(SHA256_INCR): Deleted macro.
(sha256_update): Use MD_UPDATE.
(sha256_final): Deleted function.
(sha256_write_digest): New function, replacing sha256_final, and
using MD_FINAL.
(sha256_digest): Use sha256_write_digest.
(sha224_digest): Likewise.

Rev: nettle/sha256.c:1.6