Replace Travis CI with GitHub Actions

[tomeon]

This PR introduces a GitHub Actions workflow that amounts to a transliteration of the Travis CI config.

Motivation: Travis CI is slowly fading away (?) and GitHub Actions is where all the cool kids hang out. Also, the tests seem to run quite quickly on GitHub Actions, especially in the native/x86-64 build.

Super-secret motivation: there's a GitHub action for installing Nix, which should make it relatively straightforward to support CI for the NixOS tests from #100 .

[tomeon]

Passing build is here.