Add eddsa from ref10 implementation (with some modifications).

core/crypto/ed25519/fe_0.cpp

@@ -0,0 +1,19 @@
+#include "fe.h"
+
+/*
+h = 0
+*/
+
+void fe_0(fe h)
+{
+  h[0] = 0;
+  h[1] = 0;
+  h[2] = 0;
+  h[3] = 0;
+  h[4] = 0;
+  h[5] = 0;
+  h[6] = 0;
+  h[7] = 0;
+  h[8] = 0;
+  h[9] = 0;
+}