Progress levels should use a scoped enumeration

[Amir Kamil]

Suggested definition:

enum class progress_level { internal, user };

Then a user would write progress_level::user instead of progress_level_user as currently in the spec. A scoped enumeration also prevents implicit conversions between an enumeration value and an int, resulting in better type safety.

[Amir Kamil]

Fixed in the spec in 786c9b0. However, I'll leave this open until this is fixed in the implementation.

[Amir Kamil]

Fixed in the implementation.