h_neg` takes an [`Ordering`] argument which describes the memory ordering
of this operation. All ordering modes are possible. Note that using
[`Acquire`] makes the store part of this operation [`Relaxed`], and
using [`Release`] makes the load part [`Relaxed`].

# Examples

```
use portable_atomic::{AtomicU64, Ordering};

let foo = AtomicU64::new(5);
assert_eq!(foo.fetch_neg(Ordering::Relaxed), 5);
assert_eq!(foo.load(Ordering::Relaxed), 5_u64.wrapping_neg());
assert_eq!(foo.fetch_neg(Ordering::Relaxed), 5_u64.wrapping_neg());
assert_eq!(foo.load(Ordering::Relaxed), 5);
```