add count-only method to combinations Iterator
add count-only method to combinations Iterator

as suggested on IRC.  Replaces https://github.com/rakudo/rakudo/pull/635
tbrowder/rakudo,zostay/rakudo,tony-o/rakudo,jonathanstowe/rakudo,jonathanstowe/rakudo,Gnouc/rakudo,samcv/rakudo,samcv/rakudo,awwaiid/rakudo,tbrowder/rakudo,salortiz/rakudo,b2gills/rakudo,MasterDuke17/rakudo,nbrown/rakudo,nbrown/rakudo,labster/rakudo,LLFourn/rakudo,samcv/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,paultcochrane/rakudo,ab5tract/rakudo,samcv/rakudo,azawawi/rakudo,ugexe/rakudo,rakudo/rakudo,ugexe/rakudo,dankogai/rakudo,jonathanstowe/rakudo,sjn/rakudo,softmoth/rakudo,b2gills/rakudo,sjn/rakudo,dankogai/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,usev6/rakudo,MasterDuke17/rakudo,azawawi/rakudo,ab5tract/rakudo,usev6/rakudo,tbrowder/rakudo,rakudo/rakudo,Gnouc/rakudo,sjn/rakudo,ugexe/rakudo,ungrim97/rakudo,salortiz/rakudo,raydiak/rakudo,b2gills/rakudo,MasterDuke17/rakudo,tony-o/rakudo,Gnouc/rakudo,zostay/rakudo,labster/rakudo,ugexe/rakudo,zostay/rakudo,samcv/rakudo,paultcochrane/rakudo,awwaiid/rakudo,softmoth/rakudo,usev6/rakudo,usev6/rakudo,jonathanstowe/rakudo,ab5tract/rakudo,awwaiid/rakudo,lucasbuchala/rakudo,b2gills/rakudo,Gnouc/rakudo,dankogai/rakudo,ugexe/rakudo,MasterDuke17/rakudo,paultcochrane/rakudo,rakudo/rakudo,paultcochrane/rakudo,labster/rakudo,tony-o/rakudo,tbrowder/rakudo,raydiak/rakudo,ungrim97/rakudo,lucasbuchala/rakudo,labster/rakudo,salortiz/rakudo,LLFourn/rakudo,b2gills/rakudo,raydiak/rakudo,LLFourn/rakudo,LLFourn/rakudo,tony-o/rakudo,softmoth/rakudo,salortiz/rakudo,nbrown/rakudo,awwaiid/rakudo,awwaiid/rakudo,nbrown/rakudo,raydiak/rakudo,labster/rakudo,labster/rakudo,azawawi/rakudo,jonathanstowe/rakudo,LLFourn/rakudo,zostay/rakudo,sjn/rakudo,paultcochrane/rakudo,sjn/rakudo,nbrown/rakudo,softmoth/rakudo,lucasbuchala/rakudo,salortiz/rakudo,azawawi/rakudo,Gnouc/rakudo,tony-o/rakudo,lucasbuchala/rakudo,azawawi/rakudo,ungrim97/rakudo,usev6/rakudo,ab5tract/rakudo,dankogai/rakudo,lucasbuchala/rakudo,rakudo/rakudo,ungrim97/rakudo,ab5tract/rakudo,rakudo/rakudo,Gnouc/rakudo,softmoth/rakudo,tony-o/rakudo,salortiz/rakudo,nbrown/rakudo,dankogai/rakudo,ungrim97/rakudo,rakudo/rakudo