rust: list: add `impl_list_item!` examples

There's a comprehensive example in `rust/kernel/list.rs` but it doesn't
exercise the `using ListLinksSelfPtr` variant nor the generic cases. Add
that here. Generalize `impl_has_list_links_self_ptr` to handle nested
fields in the same manner as `impl_has_list_links`.

Tested-by: Alice Ryhl <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Signed-off-by: Tamir Duberstein <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Fixed Rust < 1.82 build by enabling the `offset_of_nested`
  feature. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

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