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.

src/core/Any-iterable-methods.pm

@@ -1818,7 +1818,7 @@ Did you mean to add a stub (\{...\}) or did you mean to .classify?"
           $pulled
         )
     }
-    multi method head(Any:D: Callable:D $w) {
+    multi method head(Any:D: WhateverCode:D $w) {
         Seq.new(
            Rakudo::Iterator.AllButLastNValues(self.iterator,-($w(0).Int))
         )
@@ -1841,7 +1841,7 @@ Did you mean to add a stub (\{...\}) or did you mean to .classify?"
     multi method tail(Any:D: $n) {
         Seq.new(
           nqp::if(
-            nqp::istype($n,Callable)
+            nqp::istype($n,WhateverCode)
               && nqp::isgt_i((my $skip := -($n(0).Int)),0),
             nqp::stmts(
               (my $iterator := self.iterator).skip-at-least($skip),