Add EllipticCurve.is_infinity() which returns True when point at infi…

…nity

This is required for checks in Elligator Squared mapping functions f and r.

test/functional/test_framework/key.py

@@ -95,6 +95,13 @@ def on_curve(self, p1):
         z4 = pow(z2, 2, self.p)
         return z1 != 0 and (pow(x1, 3, self.p) + self.a * x1 * z4 + self.b * z2 * z4 - pow(y1, 2, self.p)) % self.p == 0
 
+    def is_infinity(self, p1):
+        """Return true if Jacobian tuple p is at infinity"""
+        _, _, z1 = p1
+        if z1 == 0:
+            return True
+        return False
+
     def is_x_coord(self, x):
         """Test whether x is a valid X coordinate on the curve."""
         x_3 = pow(x, 3, self.p)