Change src2/ to use BFS on ambs. Implement ambc (formerly amb-cont).

Now the following code works:
(let ((x (amb 1 2)))
  (let ((y (amb-range 0 1)))
    (if (= x 1) (amb))
    (pp (list x y))))

There's now no reason to pass failure continuations around,
and they can safely be removed from analyze.scm.

Still to do: emptying the queue on (init), hooking try-again into the
queue, and figuring out what to do when the queue is empty.