improve `derive(Zeroable)` on `union`s

[BennoLossin]

#42 added support for deriving Zeroable on unions, but it requires that all fields implement Zeroable. That is not necessary, only one field is required to implement Zeroable.

[nbdd0121]

Is this implementable?

[BennoLossin]

I think this is definitely doable with overlapping marker traits, but without them I'm not so sure

[BennoLossin]

There is assert_impl_any!, so it is possible?

[nbdd0121]

Oh Deref, mimicry's artful guise,
Wrapping types, specialization implies.
Method resolution, where the magic lies.

Disclaimer: partly AI generated