Allow null-pointer-optimized enums in FFI if their underlying representation is FFI safe

This allows types like Option<NonZeroU8> to be used in FFI without triggering the improper_ctypes lint. This works by changing the is_repr_nullable_ptr function to consider an enum E to be FFI-safe if:

- E has no explicit #[repr(...)].
- It only has two variants.
- One of those variants is empty (meaning it has no fields).
- The other variant has only one field.
- That field is one of the following:
  - &T
  - &mut T
  - extern "C" fn
  - core::num::NonZero*
  - core::ptr::NonNull<T>
  - #[repr(transparent)] struct wrapper around one of the types in this list.
- The size of E and its field are both known and are both the same size (implying E is participating in the nonnull optimization).

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