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/Collection/CollectionImpl.hpp

@@ -17,6 +17,7 @@ inline VariantSlot* CollectionData::addSlot(MemoryPool* pool) {
     return 0;
 
   if (_tail) {
+    ARDUINOJSON_ASSERT(pool->owns(_tail));  // Can't alter a linked array/object
     _tail->setNextNotNull(slot);
     _tail = slot;
   } else {