[bug#49158] Add ruby-for-crystal.

[jgart]

Hi Guix,

We've (Ryan, David, Raghav, and others) started packaging crystal for guix: 
https://crystal-lang.org/

This patch adds an old version of ruby that is required by the crystal language 
bootstrap process. This is related to 49142.

This was an effort of the volunteers at the last guix packaging meetup hosted 
by LibreMiami.

Here are some notes, questions, and a list of dependencies regarding what is 
needed to finish a properly bootstraped crystal package:

https://github.com/ryanprior/guix-packages/blob/master/testing/crystal.org

We are trying to recreate this bootstrapping process in guix:

https://github.com/crystal-lang/bootstrap-script

There are 160 stages!

Some questions extracted from our notes follow:

Is it preferable to have 160 bootstrap packages, one for each stage, or one big 
bootstrap package with 160 build-* stages, or somewhere inbetween?

Each stage needs a different checkout of the git repository - can we preserve 
info in .git such that we can checkout again during the build, or do we want to 
have each checkout be an independent input to the package?

How best can we use Guile macros to clean up the large amount of code implied 
by executing 160 stages of bootstrap logic?

best regards,

jgart

0001-gnu-Add-ruby-for-crystal.patch
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