Allow Array:: to be used as a top level namespace in modules

This basically moves the dim2role lookup from compile time to run time.
For some reason, having this lookup at compile time, caused Array:: to
be somehow special wrt serialization contexts.

This removes the error:
===SORRY!===
Object does not exist in serialization context

src/core/ShapedNArray.pm6

@@ -1,6 +1,6 @@
 # this is actually part of the Array class
 
-    constant dim2role :=
+    my \dim2role :=
       nqp::list(ShapedArray,Shaped1Array,Shaped2Array,Shaped3Array);
 
     sub set-shape(\base, \shape) is raw {