Change link() to shallowCopy() (issue #1343)

Instead of storing a pointer, the function copies the `VariantData`.

Benefits:
* smaller code
* no impact on programs that don't use this feature

Drawbacks:
* changes to the original variant are not always reflected on the copy
* modifying the original from the shallow copy leads to UB

src/ArduinoJson/Object/ObjectRef.hpp

@@ -269,12 +269,12 @@ struct Converter<ObjectConstRef> {
 
   static ObjectConstRef fromJson(VariantConstRef src) {
     const VariantData* data = getData(src);
-    return data != 0 ? data->resolve()->asObject() : 0;
+    return data != 0 ? data->asObject() : 0;
   }
 
   static bool checkJson(VariantConstRef src) {
     const VariantData* data = getData(src);
-    return data && data->resolve()->isObject();
+    return data && data->isObject();
   }
 };