re part of this operation [`Relaxed`], and
using [`Release`] makes the load part [`Relaxed`].

This function may generate more efficient code than `fetch_add` on some platforms.

- MSP430: `add` instead of disabling interrupts ({8,16}-bit atomics)

# Examples

```
use portable_atomic::{AtomicI16, Ordering};

let foo = AtomicI16::new(0);
foo.add(10, Ordering::SeqCst);
assert_eq!(foo.load(Ordering::SeqCst), 10);
```