Implement a ->digest function for sha256-ce.  This improves the
performance of crypto_shash_digest() with this algorithm by reducing the
number of indirect calls that are made.  This only adds ~112 bytes of
code, mostly for the inlined init, as the finup function is tail-called.

For now, don't bother with this for sha224, since sha224 is rarely used.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>