FIX: Use repr(C) MaybeUninitCopy after discussion with RalfJung
FIX: Use repr(C) MaybeUninitCopy after discussion with RalfJung

We have a recommendation from the unsafe-wg (no rule yet), that
repr(C) for unions should work this way, so that we can cast from the
union type to one of its fields.

bluss/arrayvec