Run tests with GitHub Actions

[pluehne]

As Travis CI seems to phase out its support of open-source projects, this introduces GitHub Actions instead. With this configuration, tests are run on Ubuntu Linux, macOS, and Windows. On Linux, both the stable and beta toolchains are tested, while tests on Windows are run for both 32- and 64-bit systems as well as both the GCC and the Microsoft Visual C++ toolchains.