st`], [`Acquire`] or [`Relaxed`].

# Panics

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

# Considerations

This method is not magic; it is not provided by the hardware.
It is implemented in terms of [`compare_exchange_weak`](Self::compare_exchange_weak),
and suffers from the same drawbacks.
In particular, this method will not circumvent the [ABA Problem].

[ABA Problem]: https://en.wikipedia.org/wiki/ABA_problem

# Examples

```
use portable_atomic::{AtomicU16, Ordering};

let x = AtomicU16::new(7);
assert_eq!(x.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |_| None), Err(7));
assert_eq!(x.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |x| Some(x + 1)), Ok(7));
assert_eq!(x.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |x| Some(x + 1)), Ok(8));
assert_eq!(x.load(Ordering::SeqCst), 9);
```