ggles the bit at the specified bit-position.

Returns `true` if the specified bit was previously set to 1.

`bit_toggle` 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`].

This corresponds to x86's `lock btc`, and the implementation calls them on x86/x86_64.

# Examples

```
use portable_atomic::{AtomicU8, Ordering};

let foo = AtomicU8::new(0b0000);
assert!(!foo.bit_toggle(0, Ordering::Relaxed));
assert_eq!(foo.load(Ordering::Relaxed), 0b0001);
assert!(foo.bit_toggle(0, Ordering::Relaxed));
assert_eq!(foo.load(Ordering::Relaxed), 0b0000);
```