Improve Experimental/do-control Terminator Semantics

[Jason94]

(Mostly dropping as a note to myself. I'll get around to this soon.)

Currently in do-control-core the semantics of the Terminator class are a little weird. The main method is called ended? which is true for True bools, and then also empty containers (None, etc.)

This leads to some weird code, like this, which makes little sense. Note how continue? has to be False to get the thread to keep going:

  ;; TODO: Flip bool and when_ term semantics
  (declare run-program-1 (IO Unit))
  (define run-program-1
    (do
     (a <- (new-tvar 0))
     (b <- (new-tvar 0))
     (continue? <- (new-mvar False))
     (thread1 <-
       (do-fork-thread_
         (do-loop-do-while (read-mvar continue?)
           (run-tx (increment-int a)))))

It should probably be reworked so that:
(1) False and empty values represent "stop," and True and non-empty values represent "continue"
(2) Existing functions that use the Terminator class should work in the more natural way. (do-when True ...) should execute, (do-loop-do-while True ...) should loop forever, etc.