slideshow/step: record disappeared bindings & uses for step names

[LiberalArtist]

trafficstars

Generally, simplify the implementation using syntax-parse.

In a separate commit, this pull request also trivially updates slideshow/step to use #lang racket/base rather than mzscheme.

[LiberalArtist]

I just hit a syntax error when trying to use a version of slideshow with this patch. I will try to track it down, but this shouldn't be merged for now.

[samth]

Would be great to get this to work, but I've converted to draft based on @LiberalArtist's most recent comment.