Should as_variant adapts to std::variant?

Currently it adapts to boost::variant, which looks either outdated or non-flexible.

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.

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.