avoid packed struct with non-Copy fields

redox_uefi is one of the few crates that was identified as part of https://github.com/rust-lang/rust/pull/104429 to have a packed struct with a built-in derive and a non-Copy field where furthermore the packed does not actually change the layout at all. (If it did change the layout, the derive would have complained about unaligned references. Usually built-in derive on packed types required derive(Copy) but there is a basically accidental exception for types where packed ends up not changing the layout at all.) This affects the types IfrCheckbox and IfrNumeric. This MR fixes that by making IfrOpHeader Copy.

Note: the types IfrCheckbox and IfrNumeric have comments saying "Has incorrect size if not packed", but I think that is wrong -- all fields of those types have alignment 1, so packed makes no difference.