do not re-export racket modules

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
fixed dependencies

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
simplify and fix the async module

Signed-off-by: Jan Dvořák <mordae@anilinux.org>
document the async module

Signed-off-by: Jan Dvořák <mordae@anilinux.org>