WIP. Would be quite useful for winit, since we don't actually need the ivars to be accessible outside Rust, and we could then avoid some unnecessary boxing.

For this to work properly we'd need some parameter that states whether the instance has been initialized or not yet (since it must be safe to dealloc an instance that has not had init called on it yet).

Other options maybe?

• Require Default and override alloc to always initialize each object?
• Require Copy (or ideally !Drop), and just forget the value (would allow native enums to work)
• Always allow it if the value is wrapped in ManuallyDrop, similar to above
• Require bytemuck::Zeroable

Fixes #414