A common Slideshow pattern is to put a subset of slides in its
own module with a `run-slides' function, and then you'd
(un)comment a `(run-slides)' call at the end of the module to
work on the subset by itself. Now, you can write
`(module+ main (run-slides))' atthe end of the module and not
have to comment it out. Adding `main' support to the `slideshow'
executable makes it more consistent with using `racket' directly.
Checking first for a `slideshow' submodule makes it possible
for `slideshow' and `racket' to do different things, in case
that's useful.
