[racket] disable garbage collection

Good points.  For my list of reasons, I was thinking only of disabling 
GC for the lifetime of a process, but there are additional reasons that 
one might want to temporarily inhibit GC for a shorter duration.

Matthias Felleisen wrote at 06/02/2011 03:29 PM:
> Recall that Disney used Chez in this way: when you're in a critical section of a ride, disable GC and allocate enough beforehand. As Kent used to say, tG, these are virtual rides. 
>   

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