[Update exercise] Crypto Square

[colinleach]

I solved this exercise in R yesterday, in preparation for adding it to Julia and Fortran. Turns out, the 8-year-old implementation diverges a lot from canonical data: over-complicated, restrictive and really quite annoying.

I've added a template, updated tests to the current problem specification, and removed all functions except ciphertext().

This should give students more freedom to explore different approaches. I don't think it breaks existing solutions, it just ignores big chunks of code.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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[colinleach]

There are 108 completions, so retesting should be reasonable.

[depial]

A bit odd that it erred with tidyverse. Any thoughts on that?

[colinleach]

My fault: sloppy copy-paste.