Add eddsa from ref10 implementation (with some modifications).

core/crypto/ed25519/fe_isnegative.cpp

@@ -0,0 +1,16 @@
+#include "fe.h"
+
+/*
+return 1 if f is in {1,3,5,...,q-2}
+return 0 if f is in {0,2,4,...,q-1}
+
+Preconditions:
+   |f| bounded by 1.1*2^26,1.1*2^25,1.1*2^26,1.1*2^25,etc.
+*/
+
+int fe_isnegative(const fe f)
+{
+  unsigned char s[32];
+  fe_tobytes(s,f);
+  return s[0] & 1;
+}