make `random` slightly faster on 64-bit machines in integer mode
make `random` slightly faster on 64-bit machines in integer mode

Exploit the fact that a 32-bit range fits in a fixnum.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket