Make List:D.iterator a multi

So that List.iterator will produce an iterator producing List, rather
than die-ing.

src/core.c/List.pm6

@@ -644,7 +644,7 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
 
         method is-lazy() { $!todo.DEFINITE && $!todo.is-lazy }
     }
-    method iterator(List:D: --> Iterator:D) {
+    multi method iterator(List:D: --> Iterator:D) {
         nqp::isconcrete($!todo)
           ?? Todo.new(self)               # something to iterate in the future
           !! nqp::isconcrete($!reified)   # everything we need is already there