Add enum class

[PopSoda2002]

Make sure to read the contributing guidelines before submitting a PR

I have tested the CI in my local computer bash ./ci/run.sh ./tmp/results ./tmp/mnt
All pass, no error.

What has been done

Change enum -> enum class, it's safer stackoverflow

[ggerganov]

This change is not useful and I don't think it improves over the way we currently use enums in the codebase. In general, enum class has advantages, but they are not very relevant to our usecases.

[PopSoda2002]

I think enum class is nothing disadvantages but advantages compared with pure enum. Sometimes pure enum will have bugs but enum class will never.