Previous/next value helpers

[turol]

There should be a way to get previous/next value in the enum, something like this:

https://github.com/turol/smaaDemo/blob/31ba838df401122afb62ebbf4c6b49c93108b011/demo/smaaDemo.cpp#L252

https://github.com/turol/smaaDemo/blob/31ba838df401122afb62ebbf4c6b49c93108b011/demo/smaaDemo.cpp#L2595

[Neargye]

For example

enum class SMAAEdgeMethod : uint8_t {
	  Color
	, Luma
	, Depth
};

nextValue(SMAAEdgeMethod::Depth) - occur error or return SMAAEdgeMethod::Color ?

https://en.cppreference.com/w/cpp/iterator/next for example throws an error (I would probably prefer to return optional)

[turol]

In my case I cycle again to the beginning. You could of course add both variants.

[turol]

On further thinking there should probably be the following variants:

1. cycle_prev/cycle_next does what I currently do.
2. prev/next does what you suggested, throws exception
3. prev_opt/next_opt returns a std::optional

This should cover every use case.

[Neargye]

Add 2 variants in master:

• cycle_prev/cycle_next does what I currently do.
• prev/next returns a std::optional

https://github.com/Neargye/magic_enum/blob/master/include/magic_enum_utility.hpp