eds.
`failure` describes the required ordering for the load operation that takes place when
the comparison fails. Using [`Acquire`] as success ordering makes the store part
of this operation [`Relaxed`], and using [`Release`] makes the successful load
[`Relaxed`]. The failure ordering can only be [`SeqCst`], [`Acquire`] or [`Relaxed`].

# Panics

Panics if `failure` is [`Release`], [`AcqRel`].

# Examples

```
use portable_atomic::{AtomicI64, Ordering};

let some_var = AtomicI64::new(5);

assert_eq!(
    some_var.compare_exchange(5, 10, Ordering::Acquire, Ordering::Relaxed),
    Ok(5),
);
assert_eq!(some_var.load(Ordering::Relaxed), 10);

assert_eq!(
    some_var.compare_exchange(6, 12, Ordering::SeqCst, Ordering::Acquire),
    Err(10),
);
assert_eq!(some_var.load(Ordering::Relaxed), 10);
```