replace List.join with an implementation that unboxes strings manually
replace List.join with an implementation that unboxes strings manually

As per the IRC discussions yesterday, this should be much faster than
letting parrot do it through the get_string vtable.
Experimental verification (or falsification) welcome.

azawawi/rakudo,rjbs/rakudo,sergot/rakudo,dankogai/rakudo,nunorc/rakudo,usev6/rakudo,rakudo/rakudo,nbrown/rakudo,paultcochrane/rakudo,tbrowder/rakudo,tony-o/deb-rakudodaily,b2gills/rakudo,Gnouc/rakudo,usev6/rakudo,tony-o/rakudo,ungrim97/rakudo,awwaiid/rakudo,MasterDuke17/rakudo,pmurias/rakudo,zhuomingliang/rakudo,labster/rakudo,Gnouc/rakudo,Gnouc/rakudo,tony-o/deb-rakudodaily,jonathanstowe/rakudo,azawawi/rakudo,awwaiid/rakudo,retupmoca/rakudo,softmoth/rakudo,zostay/rakudo,sjn/rakudo,MasterDuke17/rakudo,zostay/rakudo,salortiz/rakudo,raydiak/rakudo,skids/rakudo,samcv/rakudo,tbrowder/rakudo,labster/rakudo,zhuomingliang/rakudo,labster/rakudo,tony-o/rakudo,ungrim97/rakudo,sergot/rakudo,niner/rakudo,LLFourn/rakudo,nbrown/rakudo,cognominal/rakudo,raydiak/rakudo,LLFourn/rakudo,zostay/rakudo,tony-o/deb-rakudodaily,MasterDuke17/rakudo,tbrowder/rakudo,niner/rakudo,dwarring/rakudo,rakudo/rakudo,cygx/rakudo,Leont/rakudo,tony-o/deb-rakudodaily,ugexe/rakudo,MasterDuke17/rakudo,jonathanstowe/rakudo,ab5tract/rakudo,tony-o/deb-rakudodaily,tbrowder/rakudo,azawawi/rakudo,rakudo/rakudo,salortiz/rakudo,salortiz/rakudo,pmurias/rakudo,Gnouc/rakudo,ab5tract/rakudo,dankogai/rakudo,awwaiid/rakudo,labster/rakudo,softmoth/rakudo,jonathanstowe/rakudo,teodozjan/rakudo,Gnouc/rakudo,azawawi/rakudo,skids/rakudo,tony-o/rakudo,samcv/rakudo,usev6/rakudo,Leont/rakudo,LLFourn/rakudo,teodozjan/rakudo,teodozjan/rakudo,zhuomingliang/rakudo,sjn/rakudo,niner/rakudo,tbrowder/rakudo,dankogai/rakudo,nbrown/rakudo,b2gills/rakudo,usev6/rakudo,nunorc/rakudo,tony-o/rakudo,tony-o/deb-rakudodaily,paultcochrane/rakudo,ugexe/rakudo,rjbs/rakudo,labster/rakudo,laben/rakudo,ugexe/rakudo,softmoth/rakudo,dwarring/rakudo,lucasbuchala/rakudo,samcv/rakudo,rakudo/rakudo,dwarring/rakudo,nbrown/rakudo,ungrim97/rakudo,nbrown/rakudo,paultcochrane/rakudo,lucasbuchala/rakudo,paultcochrane/rakudo,laben/rakudo,lucasbuchala/rakudo,sergot/rakudo,ab5tract/rakudo,cygx/rakudo,samcv/rakudo,softmoth/rakudo,cognominal/rakudo,ungrim97/rakudo,tony-o/deb-rakudodaily,skids/rakudo,nbrown/rakudo,usev6/rakudo,ab5tract/rakudo,zhuomingliang/rakudo,awwaiid/rakudo,skids/rakudo,dankogai/rakudo,jonathanstowe/rakudo,sjn/rakudo,rjbs/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,pmurias/rakudo,retupmoca/rakudo,nunorc/rakudo,rakudo/rakudo,Leont/rakudo,retupmoca/rakudo,dwarring/rakudo,Gnouc/rakudo,salortiz/rakudo,azawawi/rakudo,cognominal/rakudo,jonathanstowe/rakudo,laben/rakudo,tony-o/rakudo,ab5tract/rakudo,laben/rakudo,sergot/rakudo,b2gills/rakudo,raydiak/rakudo,labster/rakudo,cygx/rakudo,tony-o/rakudo,teodozjan/rakudo,lucasbuchala/rakudo,b2gills/rakudo,sjn/rakudo,lucasbuchala/rakudo,b2gills/rakudo,cognominal/rakudo,ungrim97/rakudo,cygx/rakudo,LLFourn/rakudo,salortiz/rakudo,skids/rakudo,pmurias/rakudo,ugexe/rakudo,Leont/rakudo,softmoth/rakudo,MasterDuke17/rakudo,cygx/rakudo,salortiz/rakudo,nunorc/rakudo,retupmoca/rakudo,dankogai/rakudo,raydiak/rakudo,awwaiid/rakudo,LLFourn/rakudo,sjn/rakudo,niner/rakudo,cognominal/rakudo,ugexe/rakudo,rakudo/rakudo,paultcochrane/rakudo,zostay/rakudo,samcv/rakudo