use precompute for verification

this was originally part of the jacobi code (merged just before this
commit) but was unintentionally dropped during rebases

Author: tomato42

src/ecdsa/ellipticcurve.py

@@ -534,6 +534,10 @@ def mul_add(self, self_mul, other, other_mul):
           return other * other_mul
       if not isinstance(other, PointJacobi):
           other = PointJacobi.from_affine(other)
+      # when the points have precomputed answers, then multiplying them alone
+      # is faster (as it uses NAF)
+      if self.__precompute and other.__precompute:
+          return self * self_mul + other * other_mul
 
       if self.__order:
           self_mul = self_mul % self.__order