Fix build

[TheRealOwenRees]

Multiple changes detailed below:

• update dev dependencies to latest versions
• changes to the way the Makefile copies files from the exercise directory. This allows us to copy everything inside /src in case there are extra modules, and then overwrite the exercise file with the code from Example.re
• Exercises now need function stubs

[github-actions]

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[github-actions]

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[github-actions]

This is an unmaintained repository.

Cross-track maintainers - feel free to merge.