clean up a large memory leak

It turns out (bowboard screen 128) on a real screen massively slowed down
and ran out of memory since commit e2ab1b30b1 on May 19. The culprit was
these changes, which created memory allocations for a new trace on every
recursive call.

I originally had some vague desire to isolate these calls from the user-visible
trace. That's expensive enough that I'll wait until it becomes a concern
before trying to isolate again.

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