Support other repr layouts

[dafaust]

https://doc.rust-lang.org/reference/type-layout.html#representations

structs

• [x] repr(align)
• [x] repr(packed)
• [ ] repr(C)
• [ ] repr(transparent) - single member only
• [ ] certain combinations, e.g. repr(C, packed(2))

unions

• [ ] repr(align)
• [ ] repr(packed)
• [ ] repr(C)
• [ ] certain combinations, e.g. repr(C, packed(2))

enums

• [ ] repr(align)
• [ ] primitive reprs, e.g. repr(u8)
• [ ] repr(transparent) - single variant only
• [ ] certain combinations, e.g. repr(C, u8)

[bjorn3]

repr(transparent) allows multiple members for as long as all but one member are zero sized.

[dafaust]

Ah, good point. Thanks.