Add halt! to abort without rollback

I'm still working on a few more (and better?) use cases, but wanted to go ahead and open this to spark further discussion from where we left off in [i] campfire.  I still argue that there is a need, semantically and functionally, for a means to stop a chain of interactors in this manner.

The example is maybe a bit contrived<sup>1</sup> but at least offers another example to base discussions around.  The point is to introduce a means to stop execution of subsequent interactors in a way that is not deemed a failure and, more importantly, does not force any unwanted rollback behavior and avoids the need for successive "check" flags.  In my opinion, halt! is a valid success case.

<sup>1</sup>Also thought of a bank paying a check, but kept getting bogged down with the details in the example this early in the morning.  The gist was paybychecking -> halt! unless hasoverdraft -> paybecredit.

```ruby
class ProcessCustomerOrder
  include Interactor::Organizer
 
  organize [
 
    # No-op
    #
    # rollback
    #   - NotifyAdmin.new(message)
    NotifyException
 
    # Charge customer for purchase.
    #
    # fail!(message: oh no!)
    # rollback
    #   - give money back
    ChargeCustomer
 
    # Send now or queue for later?
    #
    # fail!(message: oh no!)
    CheckAvailableInventory
      unless inventory.available?
        CheckAvailabilityLater.enqueue(order)
        halt!(message: "Success: Ships 2-4 weeks!")
      end
 
    # Send goods to customer
    #
    # fail!(message: oh no!)
    # rollback
    # - ensure order canceled
    DeliverGoods
      if goods.deliver!
        halt!(message: "Success: Arrives 2-4 days!")
        # or (both are success cases)
        # context[:message] = "Success: Arrives 2-4 days!"
      else
        fail!(message: oh no!)
      end
 
  ]
end
 
def create
  process_order = ProcessOrder.new(order: order)
 
  if process_order.success?
    redirect_to whatever_path, notice: process_order.message
  else
    render :uh_oh
  end
end
```