Make #[derive(IntoBytes)] support more generic types

[joshlf]

Here's an idea for making #[derive(IntoBytes)] smarter on generic types: we can see from the surface syntax which fields are based on generic types and which are entirely concrete. Then, we can perform const computation based on those concrete types to extract partial information about the type. We can then use that partial information to infer bounds that must hold of generic types.

For example, if we could use const code to compute the maximum alignment that we could tolerate for a given field, then we could emit a bound of F: Unaligned<{derive_generated_computation}> for that field.

[joshlf]

Another possible avenue that doesn't require supporting Unaligned<const ALIGN: usize>: We may be able to support fully concrete, unsized #[repr(C)] types.

In particular, we can query for the sizes of all but the last field, and we may be able to compute some layout information about the last field (although this might require KnownLayout). We could then compute whether any instance of the type would contain padding.