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/Document/JsonDocument.hpp

@@ -62,7 +62,7 @@ class JsonDocument : public VariantOperators<const JsonDocument&> {
   }
 
   size_t size() const {
-    return _data.resolve()->size();
+    return _data.size();
   }
 
   bool set(const JsonDocument& src) {