message: [lang] Fix bug that ret_ty in JSON API generation is nested
diff: @@ -121,10 +121,7 @@ impl From<&hir::DeployHandler> for DeployDescription {
  /// Describes the return type of a contract message.
  #[derive(Debug, PartialEq, Eq, Deserialize, Serialize)]
  -pub struct ReturnTypeDescription {
  - /// The optional return type.
  - ret_ty: Option<TypeDescription>,
  -}
  +pub struct ReturnTypeDescription(Option<TypeDescription>);
  impl ReturnTypeDescription {
  /// Creates a new return type description from the given optional type.
  @@ -132,9 +129,7 @@ impl ReturnTypeDescription {
  where
  T: Into<Option<TypeDescription>>,
  {
  - Self {
  - ret_ty: opt_type.into(),
  - }
  + Self(opt_type.into())
  }
  }
  