Add constant-time multiply secp256k1_ecmult_const for ECDH

Designed with clear separation of the wNAF conversion, precomputation
and exponentiation (since the precomp at least we will probably want
to separate in the API for users who reuse points a lot.

Future work:
  - actually separate precomp in the API
  - do multiexp rather than single exponentiation

src/scalar_impl.h

@@ -234,6 +234,11 @@ static void secp256k1_scalar_inverse(secp256k1_scalar_t *r, const secp256k1_scal
     secp256k1_scalar_mul(r, t, &x6); /* 111111 */
 }
 
+SECP256K1_INLINE static int secp256k1_scalar_is_even(const secp256k1_scalar_t *a) {
+    /* d[0] is present and is the lowest word for all representations */
+    return !(a->d[0] & 1);
+}
+
 static void secp256k1_scalar_inverse_var(secp256k1_scalar_t *r, const secp256k1_scalar_t *x) {
 #if defined(USE_SCALAR_INV_BUILTIN)
     secp256k1_scalar_inverse(r, x);