[racket] generator performance (again)

FWIW,

1) the timings with the 32 bit version of Racket 5.3 (the other timings 
were from the 64 bit version)

fibo-clos : cpu time: 12529 real time: 12720 gc time: 850
fibo-gen1 : cpu time: 21003 real time: 22319 gc time: 1808
fibo-gen2 : cpu time: 20527 real time: 20929 gc time: 1795
fibo-delay: cpu time: 21671 real time: 21942 gc time: 9996

2) the timings from Bigloo 3.8b (just out of curiosity; no generators 
though)

fibo-clos : real: 4390 sys: 770 user: 5290
fibo-delay: real: 4490 sys: 800 user: 5380

3) for reference, the previous timings from the 64-bit version of Racket 
5.3:

fibo-clos : cpu time: 5887 real time: 6135 gc time: 716
fibo-gen1 : cpu time: 14572 real time: 15205 gc time: 1707
fibo-gen2 : cpu time: 14540 real time: 15181 gc time: 1709
fibo-delay: cpu time: 14045 real time: 14309 gc time: 8417