sc25519_mul
sc25519_mul(&scs, &scs, &scsk);
sc25519_mul(r, x, &t);
void sc25519_mul(sc25519 *r, const sc25519 *x, const sc25519 *y);
sc25519_mul(&scs, &scs, &scsk);
static void sc25519_mul(sc25519 *r, const sc25519 *x, const sc25519 *y);