Make .head and .tail only take WhateverCode
Make .head and .tail only take WhateverCode

Instead of Callable.  Being able to take a Callable is not documented
at the moment, and restricting it to WhateverCode does not break any
tests.  Should allow for more uses of Callable with .head/.tail.

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