Add function for testing quadratic residue field/group elements.

src/group.h

@@ -94,6 +94,9 @@ static void secp256k1_gej_neg(secp256k1_gej *r, const secp256k1_gej *a);
 /** Check whether a group element is the point at infinity. */
 static int secp256k1_gej_is_infinity(const secp256k1_gej *a);
 
+/** Check whether a group element's y coordinate is a quadratic residue. */
+static int secp256k1_gej_has_quad_y_var(const secp256k1_gej *a);
+
 /** Set r equal to the double of a. If rzr is not-NULL, r->z = a->z * *rzr (where infinity means an implicit z = 0).
  * a may not be zero. Constant time. */
 static void secp256k1_gej_double_nonzero(secp256k1_gej *r, const secp256k1_gej *a, secp256k1_fe *rzr);