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/MsgPack/MsgPackSerializer.hpp

@@ -56,7 +56,7 @@ class MsgPackSerializer : public Visitor<size_t> {
       writeInteger(uint32_t(n));
     }
     for (const VariantSlot* slot = array.head(); slot; slot = slot->next()) {
-      slot->data()->resolve()->accept(*this);
+      slot->data()->accept(*this);
     }
     return bytesWritten();
   }
@@ -74,7 +74,7 @@ class MsgPackSerializer : public Visitor<size_t> {
     }
     for (const VariantSlot* slot = object.head(); slot; slot = slot->next()) {
       visitString(slot->key());
-      slot->data()->resolve()->accept(*this);
+      slot->data()->accept(*this);
     }
     return bytesWritten();
   }