add custom exercise reverse-list

[oxe-i]

1. Creates and adds a custom exercise called reverse-list, which aims to prove a property about a custom list reversing function (specifically, that it is equivalent to the built-in List.reverse).
2. Changes the generator script so that it doesn't automatically fail if an exercise has no canonical-data neither extra-cases. The script outputs this information but still builds a test file with only the intro and ending of the generator.

Right now test files used by exercises do not fail on a warning. This means that sorry (which is equivalent to "always true" in proofs and only emits a warning) is hard to catch. Added an extra option in lakefile.toml so that the build fails on any warning.

I think that maybe we should add this configuration as default in the template lakefile.toml. And perhaps we should also modify the test-runner so that online tests run with --wfail (which serves the same purpose).