Kill the last use of List.ensure-allocated

And the method itself with it.  This means that now all List methods
know how to handle the case when no $!reified attribute has been
initialized, without allocating one just to be on the safe side
(which is what ensure-allocated did).

src/core/List.pm

@@ -365,10 +365,6 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
         list
     }
 
-    method !ensure-allocated(--> Nil) {
-        $!reified := nqp::create(IterationBuffer) unless $!reified.DEFINITE;
-    }
-
     multi method Bool(List:D:) {
         nqp::p6bool(
           nqp::unless(
@@ -1095,10 +1091,14 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
     }
 
     method rotor(List:D: *@cycle, :$partial) is nodal {
-        self!ensure-allocated;
         die "Must specify *how* to rotor a List"
           unless @cycle.is-lazy || @cycle;
 
+        # done if there's nothing to rotor on
+        return Rakudo::Internals.EmptyIterator
+          unless nqp::getattr(self,List,'$!reified').DEFINITE
+                   || nqp::getattr(self,List,'$!todo').DEFINITE;
+
         my $finished = 0;
         # (Note, the xx should be harmless if the cycle is already infinite by accident.)
         my @c := @cycle.is-lazy ?? @cycle !! (@cycle xx *).cache;