Redesigned JsonVariant to leverage converting constructors instead of assignment operators

test/Printers.cpp

@@ -36,3 +36,15 @@ std::ostream& ArduinoJson::operator<<(std::ostream& os,
   v.printTo(adapter);
   return os;
 }
+
+std::ostream& ArduinoJson::operator<<(
+    std::ostream& os, const ArduinoJson::JsonObjectSubscript& v) {
+  JsonVariant value = v;
+  return os << value;
+}
+
+std::ostream& ArduinoJson::operator<<(
+    std::ostream& os, const ArduinoJson::JsonArraySubscript& v) {
+  JsonVariant value = v;
+  return os << value;
+}