Clone continuation stacks on-demand

[maplant]

I noticed that the classic yin-yang puzzle[1] wasn't working properly. Turns out I was forgetting to clone the stacks! That's fixed now. Because it's an expensive operation, it's only ever done at points where scheme is given access to a first-class continuation. If you have a continuation that you would like to use, be sure to run clone_stack on it.

[1]:

(let* ((yin
         ((lambda (cc) (display "@") cc) (call-with-current-continuation (lambda (c) c))))
       (yang
         ((lambda (cc) (display "*") cc) (call-with-current-continuation (lambda (c) c)))))
    (yin yang))