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/ecdh_crypto.c

@@ -53,13 +53,11 @@ static const char *ecdh_kex_type_to_curve(enum ssh_key_exchange_e kex_type) {
 #endif /* OPENSSL_VERSION_NUMBER */
     switch (kex_type) {
     case SSH_KEX_ECDH_SHA2_NISTP256:
-#ifdef HAVE_MLKEM
     case SSH_KEX_MLKEM768NISTP256_SHA256:
-#endif
     case SSH_GSS_KEX_ECDH_NISTP256_SHA256:
         return NISTP256;
     case SSH_KEX_ECDH_SHA2_NISTP384:
-#ifdef HAVE_MLKEM
+#if HAVE_MLKEM1024
     case SSH_KEX_MLKEM1024NISTP384_SHA384:
 #endif
         return NISTP384;