Add wrapper_attr attribute to #derive(Semantics)

[ColonelThirtyTwo]

Allows adding attributes to the wrapper structs, in particular derive. For example, allows the wrappers to derive bytemuck::Pod.

Also add #[repr(transparent)] to the wrappers, since there's no reason not to and should probably have them anyway for safety.

[hadronized]

The whole semantics part is going to change a lot so I’m not quite sure this is something that is going to work afterwards. Let me have a look at whether we can merge it first and then I will start the refactoring, or the other way around.