[lang] Fix bug that ret_ty in JSON API generation is nested

lang/src/api.rs

@@ -121,20 +121,15 @@ 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.
     pub fn new<T>(opt_type: T) -> Self
     where
         T: Into<Option<TypeDescription>>,
     {
-        Self {
-            ret_ty: opt_type.into(),
-        }
+        Self(opt_type.into())
     }
 }