message: [lang] Slight refactor of return type description impl
diff: @@ -158,7 +158,10 @@ impl TryFrom<&hir::DeployHandler> for DeployDescription {
  /// Describes the return type of a contract message.
  #[derive(Debug, PartialEq, Eq, Deserialize, Serialize)]
  -pub struct ReturnTypeDescription(Option<TypeDescription>);
  +pub struct ReturnTypeDescription {
  + #[serde(flatten)]
  + opt_type: Option<TypeDescription>
  +}
  impl ReturnTypeDescription {
  /// Creates a new return type description from the given optional type.
  @@ -166,7 +169,7 @@ impl ReturnTypeDescription {
  where
  T: Into<Option<TypeDescription>>,
  {
  - Self(opt_type.into())
  + Self{ opt_type: opt_type.into() }
  }
  }
  