Streamline R:In.MERGESORT* a bit
Streamline R:In.MERGESORT* a bit

- remove one level of nqp::stmts
- make sure we always get a List with an IterationBuffer as reified
  (well, if we were given an nqp::list, it still might be an nqp::list)
- remove some unneeded Mu's

usev6/rakudo,awwaiid/rakudo,jonathanstowe/rakudo,softmoth/rakudo,Gnouc/rakudo,labster/rakudo,rakudo/rakudo,salortiz/rakudo,tony-o/rakudo,raydiak/rakudo,tbrowder/rakudo,jonathanstowe/rakudo,usev6/rakudo,Gnouc/rakudo,salortiz/rakudo,jonathanstowe/rakudo,Gnouc/rakudo,MasterDuke17/rakudo,salortiz/rakudo,tony-o/rakudo,tony-o/rakudo,lucasbuchala/rakudo,nbrown/rakudo,tbrowder/rakudo,labster/rakudo,softmoth/rakudo,usev6/rakudo,nbrown/rakudo,tbrowder/rakudo,nbrown/rakudo,jonathanstowe/rakudo,nbrown/rakudo,nbrown/rakudo,usev6/rakudo,jonathanstowe/rakudo,salortiz/rakudo,awwaiid/rakudo,lucasbuchala/rakudo,tony-o/rakudo,salortiz/rakudo,sjn/rakudo,MasterDuke17/rakudo,raydiak/rakudo,tbrowder/rakudo,lucasbuchala/rakudo,raydiak/rakudo,softmoth/rakudo,rakudo/rakudo,sjn/rakudo,rakudo/rakudo,sjn/rakudo,Gnouc/rakudo,sjn/rakudo,raydiak/rakudo,labster/rakudo,softmoth/rakudo,rakudo/rakudo,lucasbuchala/rakudo,tbrowder/rakudo,awwaiid/rakudo,lucasbuchala/rakudo,Gnouc/rakudo,tbrowder/rakudo,labster/rakudo,Gnouc/rakudo,salortiz/rakudo,labster/rakudo,rakudo/rakudo,softmoth/rakudo,nbrown/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,tony-o/rakudo,rakudo/rakudo,usev6/rakudo,MasterDuke17/rakudo,sjn/rakudo,MasterDuke17/rakudo,awwaiid/rakudo,labster/rakudo,tony-o/rakudo,awwaiid/rakudo