Streamline List.AT-POS a bit
Streamline List.AT-POS a bit

- only about 5% improvement on the fast path
- basically same slow path handling as in Array.AT-POS

awwaiid/rakudo,rakudo/rakudo,rakudo/rakudo,tony-o/rakudo,labster/rakudo,tony-o/rakudo,labster/rakudo,Gnouc/rakudo,lucasbuchala/rakudo,tony-o/rakudo,Gnouc/rakudo,tbrowder/rakudo,salortiz/rakudo,tony-o/rakudo,nbrown/rakudo,MasterDuke17/rakudo,lucasbuchala/rakudo,nbrown/rakudo,salortiz/rakudo,tbrowder/rakudo,salortiz/rakudo,tbrowder/rakudo,awwaiid/rakudo,MasterDuke17/rakudo,nbrown/rakudo,nbrown/rakudo,rakudo/rakudo,tbrowder/rakudo,nbrown/rakudo,tbrowder/rakudo,softmoth/rakudo,tony-o/rakudo,salortiz/rakudo,labster/rakudo,tbrowder/rakudo,lucasbuchala/rakudo,lucasbuchala/rakudo,labster/rakudo,Gnouc/rakudo,rakudo/rakudo,rakudo/rakudo,awwaiid/rakudo,Gnouc/rakudo,Gnouc/rakudo,labster/rakudo,softmoth/rakudo,MasterDuke17/rakudo,nbrown/rakudo,rakudo/rakudo,labster/rakudo,softmoth/rakudo,softmoth/rakudo,salortiz/rakudo,MasterDuke17/rakudo,lucasbuchala/rakudo,tony-o/rakudo,softmoth/rakudo,MasterDuke17/rakudo,salortiz/rakudo,Gnouc/rakudo,awwaiid/rakudo,awwaiid/rakudo,MasterDuke17/rakudo