The new default return_type argument is GetOwnPropertyReturnType::All,
which returns properties with both string and symbol keys (which is also
the default for [[OwnPropertyKeys]]). This means that in some cases we
need to iterate the ordered property table twice, as we don't store
string and symbol properties separately but symbols must - there's
certainly room for (performance) improvements here. On the other hand
this makes Reflect.ownKeys() return symbol properties now :^)
