Should as_variant adapts to std::variant?

[lichray]

Currently it adapts to boost::variant, which looks either outdated or non-flexible.

[nilsdeppe]

This would break C++11 incompatibility so I think making it adapt to std::variant to me seems like it would be less flexible. One option could be to have a macro flag that enables std::variant instead of boost::variant.

[jfalcou]

we can also have brigand::as_std_variant next to it and have the file dependently included.
Also you can use wrap manually if you want to anyway.