Add eddsa from ref10 implementation (with some modifications).

core/crypto/ed25519/fe_isnonzero.cpp

@@ -0,0 +1,19 @@
+#include "fe.h"
+#include "crypto_verify_32.h"
+
+/*
+return 1 if f == 0
+return 0 if f != 0
+
+Preconditions:
+   |f| bounded by 1.1*2^26,1.1*2^25,1.1*2^26,1.1*2^25,etc.
+*/
+
+static const unsigned char zero[32] = {};
+
+int fe_isnonzero(const fe f)
+{
+  unsigned char s[32];
+  fe_tobytes(s,f);
+  return crypto_verify_32(s,zero);
+}