Revert "Make List.$!reified a List instead of Mu"

This reverts commit e48fb80.

It doesn't make sense to have an attribute of type A inside type A.

src/Perl6/Metamodel/BOOTSTRAP.nqp

@@ -2826,10 +2826,10 @@ BEGIN {
     Nil.HOW.compose_repr(Nil);
 
     # class List is Cool {
-    #     has List $!reified;
+    #     has Mu $!reified;
     #     has Mu $!todo;
     List.HOW.add_parent(List, Cool);
-    List.HOW.add_attribute(List, scalar_attr('$!reified', List, List));
+    List.HOW.add_attribute(List, scalar_attr('$!reified', Mu, List));
     List.HOW.add_attribute(List, scalar_attr('$!todo', Mu, List));
     List.HOW.compose_repr(List);
 

src/core/List.pm

@@ -6,7 +6,7 @@ my class List does Iterable does Positional { # declared in BOOTSTRAP
     # class List is Cool
     #   The reified elements in the list so far (that is, those that we already
     #   have produced the values for).
-    #   has List $!reified;
+    #   has $!reified;
     #
     #   Object that reifies the rest of the list. We don't just inline it into
     #   the List class itself, because a STORE on Array can clear things and