Make Mixy (<+)/(>+) Mixy at least 5x faster
Make Mixy (<+)/(>+) Mixy at least 5x faster

- this is for worst case in identical Mixes of 26 strings
- no need to re-calculate .WHICH, so still faster for expensive .WHICHes
- shortcuts for empty Mixes and Mixes with different number of elements

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