Continuous integration

I think it would be nice to have some automatic checks on PRs, such as

* Does `build.sh` still work?
* Does `test.sh` still execute successfully?

CI should also increase the incentives to write automatic tests.

A full install takes a lot of time though, so we'd need a setup that offers sufficient CPU time. (Travis, which I'm most familiar with, has a timeout of 50 minutes).

Start by inspecting build.sh and test.sh to understand the checks CI must run and how much setup they require. Define a CI configuration that runs both scripts and supports automatic tests within the available CPU-time limit; the issue does not specify a provider or configuration file, so those choices need maintainer input.

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