Replace ARDUINOJSON_NAMESPACE with an inline namespace (#1820)

extras/conf_test/x86.cpp

@@ -9,7 +9,7 @@ static_assert(ARDUINOJSON_LITTLE_ENDIAN == 1, "ARDUINOJSON_LITTLE_ENDIAN");
 
 static_assert(ARDUINOJSON_USE_DOUBLE == 1, "ARDUINOJSON_USE_DOUBLE");
 
-static_assert(sizeof(ARDUINOJSON_NAMESPACE::VariantSlot) == 16,
+static_assert(sizeof(ArduinoJson::detail::VariantSlot) == 16,
               "sizeof(VariantSlot)");
 
 int main() {}