Set up continuous integration

I've had good experiences with [japaric/trust](https://github.com/japaric/trust), which are templates for setting up CI on Travis (Linux, macOS, BSD) and AppVeyor (Windows). They also build release binaries automatically, which is handy.

[GitHub Actions now supports CI too](https://github.blog/2019-08-08-github-actions-now-supports-ci-cd/). Since we're already using GitHub, I'm inclined to try that first, to limit the number of tools we have to sign in to and trust.

I'd also like to try making build warnings hard errors on CI, so we can avoid accidentally letting them through.