Remove visit_variant from visitor

[leostera]

The visit_variant function in the visitor record is not meaningfully used and should be removed in favor of passing around a continuation function directly.

We can start by removing it from here: https://github.com/serde-ml/serde/blob/73251df58654b232fa0d7eb1daff1f566f65396d/serde/serde.ml#L400-L404

And following the types. The end result should be that the data deserializers, like JSON, take visitor that is really a continuation we can call De.deserialize on:

- let deserialize_variant self { reader; _ } visitor ~name:_ ~variants:_ =
+ let deserialize_variant self { reader; _ } de ~name:_ ~variants:_ =
    Parser.skip_space reader;
    match Parser.peek reader with
    | Some '{' ->
        let* () = Parser.read_object_start reader in
        Parser.skip_space reader;
- let* value = Visitor.visit_variant self visitor in
+ let* value = De.deserialize self de in
        Parser.skip_space reader;
        let* () = Parser.read_object_end reader in
        Ok value
    | Some '"' -> Visitor.visit_variant self visitor
    | _ -> assert false

[linear[bot]]

RIOT-23 Remove visit_variant from visitor