Make ErrorType a RustEnum, and flag it as must_use

[EvanCarroll]

#5

[lu-zero]

Probably we should setup a CI, are you sure this PR works as it is?

[EvanCarroll]

@lu-zero The test worked. ;) I'm happy to set up CI, if we're going down that route though -- we should probably move to GitLab too. As that's where the upstream project is and the CI on GitLab is way better. I can set up that up if you'd like.

[lu-zero]

We have a twin organization on gitlab for this purpose, I didn't setup a syncronizer yet, but if you like I'll add you there as well.

[EvanCarroll]

@lu-zero your call! =) When you say "twin organization" i take it to mean you're already set up with GitLab and interested in the migration from GitHub to GitLab. If you want me to handle that, happy to. Alternatively, if you want to keep the projects on GitHub I can look into CI here too, but I have had a lot more experience with GitLab.

[lu-zero]

We have also an org on Gitlab and my plan is to keep both so who prefers gitlab can keep working on gitlab (e.g. @tmatth is developing a qoi impl there) or github and we hopefully can sync the trees as needed.

Since you prefer using gitlab I'm fine with moving the development of svt-av1-rs there, I invited you already there as well. :)