remove `rackunit' dependency on `srfi/13'
remove `rackunit' dependency on `srfi/13'

Cuts the bytecode footprint of `rackunit' in half
(by more than 200 KB).

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