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/Array/ElementProxy.hpp

@@ -106,8 +106,8 @@ class ElementProxy : public VariantOperators<ElementProxy<TArray> >,
     return getOrAddUpstreamElement().template to<T>();
   }
 
-  FORCE_INLINE void link(VariantConstRef value) const {
-    getOrAddUpstreamElement().link(value);
+  FORCE_INLINE void shallowCopy(VariantConstRef value) const {
+    getOrAddUpstreamElement().shallowCopy(value);
   }
 
   // Replaces the value