Flesh out GitHub Actions CI

[4ment]

travis-ci.org does not build anything since June 15

[matsen]

Suggesting that we move to GitHub Actions.

One amazing thing about this is that they can test on OS X:
https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners

But for this we'll probably save ourselves some pain by getting through #356 first.

[matsen]

Let's have both gcc and clang builds.

[matsen]

Also add a badge for GH actions CI, and add into the PR process.

[matsen]

I note that we should really revise the README... people don't have to install BEAGLE.