Don't store string literals by pointer anymore

Fixes #2189

extras/tests/JsonDocument/subscript.cpp

@@ -114,6 +114,7 @@ TEST_CASE("JsonDocument::operator[] key storage") {
     REQUIRE(doc.as<std::string>() == "{\"hello\":0}");
     REQUIRE(spy.log() == AllocatorLog{
                              Allocate(sizeofPool()),
+                             Allocate(sizeofString("hello")),
                          });
   }