Mark sub infix:<=> as an "only" sub
Mark sub infix:<=> as an "only" sub

Assignment is handled directly in the compiler.  Add some comments to this
effect and make it an only to mark its "specialness" in that respect.

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