for consistency, use FastTrack() builder instead of subclassing

manually.
this allows to introduce compile-time optimization.

Author: apotonick

lib/trailblazer/macro.rb

@@ -46,7 +46,8 @@ def self.task_adapter_for_decider(decider_with_step_interface, variable_name:)
     def self.block_activity_for(block_activity, &block)
       return block_activity, block_activity.to_h[:outputs] unless block_given?
 
-      block_activity = Class.new(Activity::FastTrack, &block) # TODO: use Wrap() logic!
+      # block_activity = Class.new(Activity::FastTrack, &block) # TODO: use Wrap() logic!
+      block_activity = Activity.FastTrack(&block) # TODO: use Wrap() logic!
       block_activity.extend Each::Transitive
 
       return block_activity, block_activity.to_h[:outputs]