message: [ocaml-gen] use [@@boxed] for single field tuple
diff: @@ -521,10 +521,10 @@ pub fn derive_ocaml_gen(item: TokenStream) -> TokenStream {
  // this will produce something like this:
  //
  // ```
  - // type ('field) scalar_challenge = { inner: 'field }
  + // type ('field) scalar_challenge = { inner: 'field } [@@boxed]
  // ```
  if generics_ocaml.len() == 1 {
  - format!("{{ inner: {} }}", generics_ocaml[0])
  + format!("{{ inner: {} }} [@@boxed]", generics_ocaml[0])
  } else {
  generics_ocaml.join(" * ")
  }
  