Circular structure printing

[Scott E. Fahlman <Fahlman at Cmu-20c>]

The inefficiency that you point to is precisely the reason we hung
circular printing under the PRINCIRCLE switch.  If you turn that on,
printing becomes an expensive process, but if you need it, it's worth
the price.

Actually, the right move when this switch is on is probably to just go
ahead and print the stuff normally, looking for circularities as you go,
but into a string rather than out to the screen.  If you make it
through, just dump the string; if you hit a circularity, go back and do
it over.  Then, when the switch is on, it only costs you maybe a factor
of two unless you do in fact hit a circularity.

-- Scott