Guarantee that it is sound to observe the bytes of None::<P> where P is a pointer type subject to NPO

[joshlf]

In #115333, we added a guarantee that transmuting from [u8; N] to Option<P> is sound where P is a pointer type subject to the null pointer optimization (NPO). It would be useful to be able to guarantee the inverse - that if all of the bytes of P are initialized, then all of the bytes of None::<P> (and thus all of the bytes of any Option<P>) are initialized. For example, this would allow zerocopy to support safe transmutation from Option<P> to [u8; N] (google/zerocopy#596).

I'm opening this issue first rather than a PR so there's an opportunity to discuss whether this is something we want, how it would be best to document it, etc.