Make VariantSlot a union. Include next slot id in VariantData

extras/tests/ResourceManager/size.cpp

@@ -3,6 +3,7 @@
 // MIT License
 
 #include <ArduinoJson/Memory/ResourceManager.hpp>
+#include <ArduinoJson/Memory/ResourceManagerImpl.hpp>
 #include <ArduinoJson/Memory/VariantPoolImpl.hpp>
 #include <catch.hpp>
 
@@ -21,10 +22,10 @@ TEST_CASE("ResourceManager::size()") {
   SECTION("Doesn't grow when allocation of second pool fails") {
     timebomb.setCountdown(1);
     for (size_t i = 0; i < ARDUINOJSON_POOL_CAPACITY; i++)
-      resources.allocSlot();
+      resources.allocVariant();
     size_t size = resources.size();
 
-    resources.allocSlot();
+    resources.allocVariant();
 
     REQUIRE(size == resources.size());
   }