Add scalar blinding and a secp256k1_context_randomize() call.

This computes (n-b)G + bG with random value b, in place of nG in
 ecmult_gen() for signing.

This is intended to reduce exposure to potential power/EMI sidechannels
 during signing and pubkey generation by blinding the secret value with
 another value which is hopefully unknown to the attacker.

It may not be very helpful if the attacker is able to observe the setup
 or if even the scalar addition has an unacceptable leak, but it has low
 overhead in any case and the security should be purely additive on top
 of the existing defenses against sidechannels.

src/group_impl.h

@@ -396,6 +396,17 @@ static void secp256k1_gej_add_ge(secp256k1_gej_t *r, const secp256k1_gej_t *a, c
     r->infinity = infinity;
 }
 
+static void secp256k1_gej_rescale(secp256k1_gej_t *r, const secp256k1_fe_t *s) {
+    /* Operations: 4 mul, 1 sqr */
+    secp256k1_fe_t zz;
+    VERIFY_CHECK(!secp256k1_fe_is_zero(s));
+    secp256k1_fe_sqr(&zz, s);
+    secp256k1_fe_mul(&r->x, &r->x, &zz);                /* r->x *= s^2 */
+    secp256k1_fe_mul(&r->y, &r->y, &zz);
+    secp256k1_fe_mul(&r->y, &r->y, s);                  /* r->y *= s^3 */
+    secp256k1_fe_mul(&r->z, &r->z, s);                  /* r->z *= s   */
+}
+
 static void secp256k1_ge_to_storage(secp256k1_ge_storage_t *r, const secp256k1_ge_t *a) {
     secp256k1_fe_t x, y;
     VERIFY_CHECK(!a->infinity);