Update discriminant-elision optimization on Option-like enums

[gnzlbg]

This PR updates the discriminant-elision optimization on option-like enums with the semantics specified in rust-lang/rust#60300

The main change is requiring that option-like enums do not have any explicit #[repr(...)] attributes.

[RalfJung]

This still does not specify, though, that transmuting between e.g. u32 and Option<NonZeroU32> is safe, right? For that we have to say that the Some case of that option is represented exactly like the inner type, and the None case is 0. I guess this works for types that have a "single-value niche", which is the case for the NonZero*/NonNull types. For &T, their niche currently is just 0, but in the future might grow to also include some unaligned values (e.g., &u32 could have a niche consisting of the values 0..=3).

EDIT: Oh I see, that RFC pretty much fixes the None value to be 0. But where do our documents state that?

[gnzlbg]

The last commit hyperlinks everything.

[RalfJung]

LGTM, thanks!