Support std::variant

Hello. I have a problem with using std::variant with boost::format.
Visual Studio 2019 - 16.7.3
boost - 1.74

Sample code:

`
#include < iostream>
#include < variant>
#include
#include

typedef std::variant MemCaptureIdType;
//typedef boost::variant MemCaptureIdType;

std::ostream& operator<<(std::ostream& os, const MemCaptureIdType& type)
{
return os;
}

int main()
{
auto tmp1 = MemCaptureIdType(0);

auto tmp = boost::format("%1%") % tmp1;

return 0;
}
`

But it works when I use boost::variant.
I think we need to support std::variant.