Make AtomicRef::new const fn

[yvt]

Turns AtomicRef::new into const fn, making it callable in constant contexts. This currently requires the use of a nightly-only feature, so it's feature-gated by the Cargo feature nightly. This can be removed in the future when this unstable feature is stabilized.

Raises the minimum supported Rust version to 1.32.0 (regardless of the nightly feature).
As of 5e5a057, this PR raises the minimum supported Rust version to 1.46.0.

This PR depends on #1.

[mystor]

This feature is on-route to being stabilized in the near future (rust-lang/rust#72437). It might be nice to avoid the nightly feature here by waiting for that feature to be stabilized first, and releasing a 0.2.0 which requires that rustc version, and gets rid of unnecessary APIs like static_atomic_ref!.

If you need const constructors right now, it might be easier to add:

const fn null() -> Self {
    AtomicRef {
        data: AtomicPtr::new(ptr::null_mut()),
        _marker: PhantomData,
    }
}

const fn from(r: &'a T) -> Self {
    AtomicRef {
        data: AtomicPtr::new(r as *const T as *mut T),
        _marker: PhantomData,
    }
}

[yvt]

I added commits that got rid of nightly feature since const_if_match was stabilized in 1.46.0.

[mystor]

Thanks!