adjust `case' expansion to work with Typed Racket
adjust `case' expansion to work with Typed Racket

Relies on the new ad hoc optimization of `hash-ref' to preserve
performance.

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