[boost] variant with shrink

boost::variant should be variant<int,int> => int variant<int,double> => double variant<int,boost::rational<int> > => boost::rational<int> variant<int,double,std::string> => variant<double,std::string> I wrote such extension https://github.com/niitsuma/variant_shrink Default generalize order is int < boost::rational<int> < float < double < std::complex<double> You can use other custom generalize relation by defining function object is_generalizable_to<Type,TypeTo>