TravisCI replacement urgently needed

[Coolthulhu]

New pricing plan for TraviCI makes it impossible to use it as the primary build system for BN. They supposedly allow applying for OSS credits, but the general sentiment seems to be that it takes very long time for them to respond, if they ever respond at all.
https://travis-ci.community/t/intending-to-use-travis-for-a-foss-project-forget-it/11406

The most likely solution will be Github Actions, as everyone recommends it and DDA uses it.
If porting doesn't go well, I'll try https://github.com/marketplace/actions/run-travis-yml

I'll try to switch over over the weekend. Until it's done, I probably won't merge PRs.

[Coolthulhu]

Didn't fix yet...

[Coolthulhu]

Seems to be working now, after porting a bunch of fixes and disabling -Wrange-loop-analysis.

[Coolthulhu]

Oh wait, that's just releases. Still need to port PR analysis.