Enable Cirrus CI

[tobim]

• Add macOS build
• Prettify names

[mavam]

The style check seems to fail despite the checkmark:

git diff -U0 --no-color $(git merge-base origin/master HEAD) | scripts/clang-format-diff.py -p1;
fatal: Not a valid object name origin/master

[tobim]

The style check seems to fail despite the checkmark:

git diff -U0 --no-color $(git merge-base origin/master HEAD) | scripts/clang-format-diff.py -p1;
fatal: Not a valid object name origin/master

Thanks for pointing that out, it is fixed now.

[tobim]

To get really nice names we'd have to stop using the matrix feature, because the option alternatives are always added to the name field. This would not be extremely painful, because of the yaml anchor feature, but still a bit more repetitive than what we have now.
As for X_task overriding the name key: This is a known cirrus bug and will sort itself out.

In all, I consider this PR complete.

[mavam]

Great work!