lib/crypto: sha256: Make library API use strongly-typed contexts

Currently the SHA-224 and SHA-256 library functions can be mixed
arbitrarily, even in ways that are incorrect, for example using
sha224_init() and sha256_final().  This is because they operate on the
same structure, sha256_state.

Introduce stronger typing, as I did for SHA-384 and SHA-512.

Also as I did for SHA-384 and SHA-512, use the names *_ctx instead of
*_state.  The *_ctx names have the following small benefits:

- They're shorter.
- They avoid an ambiguity with the compression function state.
- They're consistent with the well-known OpenSSL API.
- Users usually name the variable 'sctx' anyway, which suggests that
  *_ctx would be the more natural name for the actual struct.

Therefore: update the SHA-224 and SHA-256 APIs, implementation, and
calling code accordingly.

In the new structs, also strongly-type the compression function state.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250630160645.3198-7-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>