Switching to GitHub actions

[zesterer]

@mvdnes I'm switching the repository CI over to GitHub actions. This will make CI automatically integrated with PRs and forks. I'm also switching the documentation in Cargo.toml over to the default docs.rs domain to avoid those using stable releases being redirected to unstable docs. I will, however, keep the documentation badge that redirects to your own URL to allow nightly docs to work fine.

[mvdnes]

I am fine with this. Good idea.

My own docs only update when Travis is run, and will only publish the stable docs. If you want to remove this, and change the docs path, this is fine with me as well.