Added Github Actions pipeline

[berendsliedrecht]

• Runs a CI pipeline for incoming PRs and merges to main.
  • audit
  • build
  • check
  • clippy
  • docs
  • format
  • tests
• Removed some files that I assumed are not used (please give this a careful review as I am not fully aware of the history of these files).
• Does not include the release to crates.io just yet.
• Moved the ursa_ packages to an Ursa folder to keep the root a bit cleaner. They kept the ursa_ prefix as we might want to release them as seperate packages, although some of them are empty.

[brentzundel]

Is this still a draft?

[berendsliedrecht]

I think I left it in a draft as it still misses the release to crates.io. I also wasn't sure, or able to find, what the continuous integration from appveyor is doing exactly. I think a couple of small additions, making releases possible, would complete the PR.