Allow initialization using a const fn, and initialization to values other than None

[thomcc]

Hi. This seems to work, and somewhat invalidates the need for the macro too, although I left it and just used static_none to initialize it instead. I don't mind reverting the macro changes if you want -- They are not fully backwards compatible, if someone did &*THE_STATIC_FROM_THE_MACRO it would work before (invoke the Deref of the dummy type explicitly) but not after.

I wasn't sure what to name the functions, and don't really what I chose a lot, but maybe they're fine. If you'd prefer different names let me know.

The move to AtomicPtr for AtomicRef::static_some is needed because pointer to int in const fn isn't allowed. I kind of doubt it ever will be allowed, given that a relocation will need to be emitted for constructs like this in dynamic libraries on some platforms, and usize completely obscures that (I was a little surprised it worked with AtomicPtr, to be honest.

I think if you wanted you could clean this up a bit now, since the macro and the ATOMIC_U8_REF_INIT are both a little redundant. The crate-level docs probably need updating too since this maybe is a more ergonomic API than the one they show, but I can do that if you want, or I can leave it to you.

[thomcc]

I also wonder if it would be worth providing a different type for an AtomicRef that's not allowed to be None, now that you can provide a static initializer.

[thomcc]

Oh, hm, I missed the other PR, which does this too. I guess it's a bit redundant if conditionals in const fn are that close to stabilizing.

[mystor]

I think I'm going to close this in favour of #2. Thanks for the PR though!