Free implementation of srfi/5

[LiberalArtist]

Replace non-free syntax-rules implementation of SRFI 5
with a free implementation using syntax-parse, which
should also give better error messages.

Add some tests that both the old and new implementations pass.

Deprecate the srfi-lib-nonfree package, but retain it
as a stub for backwards compatibility.

[LiberalArtist]

I'd appreciate someone checking that I've done the right things with the package dependencies and implies statements, in particular.

[LiberalArtist]

In this PR, I only change the docs by using the same message for SRFI 5 as for SRFI 29.

I think it would be nice to do more, but I'm not completely settled on what the right thing is or how to implement it in Scribble. Currently, the free srfi-doc documentation doesn't even mention the existence of the srfi-doc-nonfree docs, which seems confusing, especially since srfi-doc reads like it's a comprehensive list of SRFIs in Racket and, before #5, it was.

For people who don't have srfi-doc-nonfree, I think the best experience would be for links (especially from right-click > "View documentation for …") to point to a section of the free SRFI Scribble document. We could explain the license for the original spec in prose and resolve the link either to the copy from srfi-doc-nonfree, if it's installed, or otherwise to https://docs.racket-lang.org. I think Scribble has the infrastructure to do this, though I haven't yet tried using the 'indirect-link style property with something other than seclink.

This would be a change, though: currently, "View documentation for …" links try to jump into the original spec. I haven't looked into how they do this, and, at least for SRFI 5, it doesn't seem to work very well right now (i.e. the link goes to a badly-chosen place in the spec). I don't know if this change would bother anyone. It might be more relevant for SRFI 29, which defines multiple bindings.

I think it would also be reasonable to keep things very simple and just link to the versions on https://srfi.schemers.org for these two SRFIs, but that would be an even bigger change.

[LiberalArtist]

Related to #4

[samth]

This looks good to me.