data_privacy: add support for more type's kinds through `#[classified]`

[Vaiz]

#[classified] macro works strictly on tuples and rejects any other type which forces to write code in a certain way.

Suggestions:

• add support for structs with only one field
• consider adding support for enums

[geeknoid]

#[classified] works for structs and tuple structs already, you can see this use in examples and tests. It indeed doesn't support enums.

Aside from enums, can you show an example of what doesn't work that you'd like to see working?

[Vaiz]

it doesn't work for structs with named fields, even if there is only one field

error: Named fields aren't supported
188 |       struct Token {
    |  __________________^
189 | |         value: String,
190 | |     }
    | |_____^