Use reinterpret_sized for array translation in unions

[lucic71]

Box<[T]>::byte_size cannot be implemented because [T] is not Sized. For dynamic types, like Box<[T]> use Ptr::reinterpret_sized(usize) instead of Ptr::reinterpret_cast(). The usize argument of reinterpret_sized encodes the C size of the dynamic Rust object.

Internally, reinterpret_cast calls reinterpret_sized(U::byte_size()).

Box<[T]>::to_bytes and Box<[T]>::from_bytes can be implemented and they are required in order to read and write a reinterpreted array.

[nunoplopes]

I'm not following this set of patches.
Shouldn't raw_()return Ptr<u8> instead of Ptr<Box<[u8]>>?

The pointer can access the byte size. Rust knows the size, so I don't understand why we need to store it ourselves.

[lucic71]

I'm not following this set of patches.

After #205 in which I translated unions as a byte array + Ptr accessors, I focused on adding ByteRepr for types that can take part in unions: enums, structs, pointers, arrays.

The current PR adds translation for array members. But I think I over-engineered this. Ptr<T> instead of Ptr<Box<[u8]>> is enough. My reasoning was that the accessor must return a pointer to the declared member type. But in the case of arrays, returning Ptr<T> is easier and actually faster because Ptr<T> doesn't have to materialize the entire array, just one element.

Shouldn't raw_()return Ptr<u8> instead of Ptr<Box<[u8]>>?

The pointer can access the byte size. Rust knows the size, so I don't understand why we need to store it ourselves.

I will close this PR and add one more small commit over #218 to have array members translated as Ptr<T>