Add eddsa from ref10 implementation (with some modifications).

core/crypto/ed25519/keypair.cpp

@@ -0,0 +1,20 @@
+#include <string.h>
+#include "crypto_sign.h"
+#include "crypto_hash_sha512.h"
+#include "ge.h"
+
+int crypto_sign_pubkey(unsigned char*pk, const unsigned char* sk) 
+{
+  unsigned char az[64];
+  ge_p3 A;
+
+  crypto_hash_sha512(az,sk,32);
+  az[0] &= 248;
+  az[31] &= 63;
+  az[31] |= 64;
+
+  ge_scalarmult_base(&A,az);
+  ge_p3_tobytes(pk,&A);
+
+  return 0;
+}