Access to flag_to_string function

[D00Movenok]

Hi there!

I'm using your lib with external logging. For nicer logs representation flag_to_string function needs to be public.

To solve this problem I found two ways:

1. Make it VM::flag_to_string, not VM::util::flag_to_string and make it public;
2. Make VM::util structure public.

Because it is your project, going to ask you, which one do you prefer? The first one seems better to me. Will PR according to your answer :)

Also it seems that macros from boost/preprocessor.hpp (https://stackoverflow.com/a/5094430) may be used to automate the headache of creating such a big switch-case. Is it ok for you, if I'll try to rewrite the func?

[kernelwernel]

the thing is, that function was meant only for debug purposes to convert the uint8_t enum values into the corresponding technique as a string because of how annoying it was for me to do that manually.

I also don't really want the util struct to be public because it was never meant to be public in the first place, it's mostly meant to provide shared functionalities for the techniques. So that's a no go for me.

I'm also not really keen with the idea of adding boost, because that's going to require people to download it, making the library not ready out of the box anymore, which I highly value. And plus I'd say it's not worth sacrificing the default compatibility of the lib for all systems by adding boost as a requirement just for a debug feature. So that's also a no-go for me again.

I can definitely make VM::flag_to_string() in a public context, but I sort of have a strict guideline for myself to carefully pick what should be made public, which I'll add in probably under an hour.

Thanks for the recommendation though!

[kernelwernel]

@D00Movenok done: #134

I removed the "VM::" parts in the output because I assumed that was a bit redundant. Let me know if anything else is needed :)