[racket] hooray for JIT

I just realized/remembered what a big win the Racket JIT is.  Thank you 
for JIT!

(After hand-optimizing some legacy code today, speeding it up 7x, I 
thought I'd disable the JIT, to get the profiler to drill down past some 
inlining in the remaining hotspot.  I disabled the JIT, ran the code, 
and then waited, and waited, and went to the kitchen and started making 
a split pea soup, and waited.... Looks like the Racket 5.3.4 JIT alone 
is giving us slightly over 20x speedup on this particular 
performance-sensitive code.  Some of the code in this system was written 
for PLT 103 or 200, so the JIT coming along sometime after that has 
probably been a lifesaver as the system grew.)

Neil V.