Make use of strongly typed enums (enum class)

[mackal]

All our supported compilers support strongly typed enums now, so we should make use of them where appropriate.

These provide stronger type safety which also means we may need to use some casts in comparisons. See: http://www.stroustrup.com/C++11FAQ.html#enum And an example: f9a7da61b8387a6496d167171ebdcfc8635dae0d