Add missing AbstractStruct.[] alias for new

Author: pitr-ch

lib/concurrent/synchronization/abstract_struct.rb

@@ -152,6 +152,7 @@ def ns_initialize(*values)
           end
         end
         clazz.class_exec(&block) unless block.nil?
+        clazz.singleton_class.send :alias_method, :[], :new
         clazz
       end
     end