#[derive(IntoBytes)] on unions should not consider #[repr(packed)] sufficient to guarantee layout

[joshlf]

This is a soundness hole. #[repr(packed)] is a modifier to other reprs, but does not on its own guarantee layout, and does not require Rust to promise not to perform niche-filling optimizations.

While we're here, we should audit other locations where we permit repr(packed) without repr(C).

[joshlf]

Update: we've decided this isn't a problem since we don't support unions with generics, and so we always emit a padding check. Since union fields are entire types (unlike enum variants, which can have sub-fields), our existing union padding check is sufficient.