fe25519_cmov
void fe25519_cmov(fe25519 *r, const fe25519 *x, unsigned char b);
fe25519_cmov(&r->x, &p->x, b);
fe25519_cmov(&r->y, &p->y, b);
fe25519_cmov(&t->x, &v, negative(b));
static void fe25519_cmov(fe25519 *r, const fe25519 *x, unsigned char b);
fe25519_cmov(&r->x, &p->x, b);
fe25519_cmov(&r->y, &p->y, b);
fe25519_cmov(&t->x, &v, negative(b));