Native ML-KEM768 implementation

for cryptographic backends that do not have support for ML-KEM (old
OpenSSL and Gcrypt; MbedTLS).

Based on the libcrux implementation used in OpenSSH, taken from this
revision:

https://github.com/openssh/openssh-portable/blob/6aba700/libcrux_mlkem768_sha3.h

But refactored to separate C and header file to support testing and
removed unused functions (to make compiler happy).

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Pavol Žáčik <pzacik@redhat.com>

src/mlkem.c

@@ -30,8 +30,10 @@ const struct mlkem_type_info *kex_type_to_mlkem_info(enum ssh_key_exchange_e kex
     case SSH_KEX_MLKEM768X25519_SHA256:
     case SSH_KEX_MLKEM768NISTP256_SHA256:
         return &MLKEM768_INFO;
+#ifdef HAVE_MLKEM1024
     case SSH_KEX_MLKEM1024NISTP384_SHA384:
         return &MLKEM1024_INFO;
+#endif
     default:
         return NULL;
     }