switch `generator' implementation to use `prompt' and `call/cc'
switch `generator' implementation to use `prompt' and `call/cc'

Instead of `shift' and `reset'. Performance is a little better,
since `call/cc' acts as hint to the run-time system that the
continuation doesn't need to compose.

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