Feature-gate RAR support

[cyqsimon]

See #565.

This PR does not fully close the issue; the ideal solution would be to use a FOSS library for unrar support. But that can wait.

Unresolved/todo

• Should unrar feature be opt-in or opt-out?
  • I personally prefer opt-in, again because of unrar's unfriendly license.
• Update CI to test both cases (with unrar feature enabled and disabled).
• Changelog entry.

[marcospb19]

• Should unrar feature be opt-in or opt-out?

• I personally prefer opt-in, again because of unrar's unfriendly license.

I'd prefer if it was opt-out, here's how I think about it:

• If you have a restriction in your distribution, you can restrict this feature so you're good 👍.

Instead of:

• If you don't have any restrictions related to this problem that other people might have, you can add this feature.

I think the first one seems more natural, plus, Ouch is licensed under MIT so people should be aware that non-FOSS code might be in it, at least if that's a dealbreaker for them.

[cyqsimon]

I'd prefer if it was opt-out, here's how I think about it:

• If you have a restriction in your distribution, you can restrict this feature so you're good 👍.

Yeah that's fine. I'm happy to respect project owners' decision on this.

[cyqsimon]

Rebased.

[marcospb19]

Hi! Is it ready for review?

PR is set as draft.

[cyqsimon]

Hi! Is it ready for review?

PR is set as draft.

Not yet. Still have to modify CI (which could really use some cleaning up). I'll also try to reenable tests on Windows and see what's wrong there.

Will make some time for this tomorrow.

[cyqsimon]

Blocking on #578.

[marcospb19]

Yep sorry for that delay, I'll answer you as soon as feasible.

[cyqsimon]

Rebased on main.

[marcospb19]

Thanks for this, I gave it a deep review but found no flaws.

I also appreciate your help with the CI and with how to use insta with conditional compilation, I never thought of changing the test name (I didn't know it was possible, hard to find it in the docs).