Cleanup macros around `Zeroable` & `ZeroableOption`

[BennoLossin]

The code around the Zeroable and ZeroableOption traits in src/lib.rs has evolved and can benefit from some cleanup for clarity and maintainability.

1. Reorder definitions and implementations

   • Ensure consistency in placement: put the Zeroable trait and its implementations (including macro invocations) first in the relevant section of src/lib.rs, followed by the ZeroableOption trait and its implementations.
   • This includes all macro definitions & uses related to these traits.

2. Rename macro for clarity

   • One of the macros is currently named impl_non_zero_int_zeroable_option; it should be renamed to impl_zeroable_option for consistency.

3. Macro usage for generic impls

   • Use the impl_zeroable_option macro to handle generic impls for types like &T, &mut T, NonNull<T>, and others (for which Option<T> is guaranteed to be zeroable).
   • This requires modifying the macro to support generics like impl_zeroable.

Please make sure your changes abide by the Linux kernel's contribution rules. Before submitting a PR, read and follow the guidelines in CONTRIBUTING.md.