crypto: blake2b - sync with blake2s implementation

Sync the BLAKE2b code with the BLAKE2s code as much as possible:

- Move a lot of code into new headers <crypto/blake2b.h> and
<crypto/internal/blake2b.h>, and adjust it to be like the
corresponding BLAKE2s code, i.e. like <crypto/blake2s.h> and
<crypto/internal/blake2s.h>.

- Rename constants, e.g. BLAKE2B_*_DIGEST_SIZE => BLAKE2B_*_HASH_SIZE.

- Use a macro BLAKE2B_ALG() to define the shash_alg structs.

- Export blake2b_compress_generic() for use as a fallback.

This makes it much easier to add optimized implementations of BLAKE2b,
as optimized implementations can use the helper functions
crypto_blake2b_{setkey,init,update,final}() and
blake2b_compress_generic(). The ARM implementation will use these.

But this change is also helpful because it eliminates unnecessary
differences between the BLAKE2b and BLAKE2s code, so that the same
improvements can easily be made to both. (The two algorithms are
basically identical, except for the word size and constants.) It also
makes it straightforward to add a library API for BLAKE2b in the future
if/when it's needed.

This change does make the BLAKE2b code slightly more complicated than it
needs to be, as it doesn't actually provide a library API yet. For
example, __blake2b_update() doesn't really need to exist yet; it could
just be inlined into crypto_blake2b_update(). But I believe this is
outweighed by the benefits of keeping the code in sync.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>