Add Atomic::const_null

[taiki-e]

Inspired by parking_lot's const_mutex and Mutex::const_new.

This allows creating a null atomic pointer in a constant context on stable Rust.

The nightly feature is still needed to create a null atomic pointer to ?Sized + Pointable types implemented by the user in a const context, but Atomic::const_null will probably be enough for many users.

[taiki-e]

Hmm, if we support both Atomic<T>::const_null and Atomic<[MaybeUninit<T>]>::const_null, the user need to specify type to disambiguate. (like, Atomic::<u8>::const_null(), Atomic::<[MaybeUninit<_>]>::const_null())

[taiki-e]

closing in favor of #797.