Rustdoc-Json: Store number of fields for tuple struct

[aDotInTheVoid]

In my mind what should happen to Struct is we put fields in StructType, eg

enum StructKind {
   Unit,
   Tuple(Vec<Option<Id>>),
   Normal {
       fields: Vec<Id>,
       fields_stripped: bool,
  },
}

That way we only show the position of #[doc(hidden)] in the tuple case where it's relevant.

If that sounds good I'll send a follow up PR

Originally posted by @aDotInTheVoid in #101462 (comment)

@rustbot claim

@rustbot modify labels +A-rustdoc-json +T-rustdoc