Merge branch 'gitlab-ci' into 'master'

Set up GitLab CI

This merge request is now ready. Build with gcc and clang works, all tests pass except tests.aconversion.sh (see #83). The last complete build is [pipelines/917](https://gitlab.fit.cvut.cz/travnja3/automata-library/pipelines/917).

Jobs may sometimes fail due to not enough space on disk, I’m gonna fix this problem soon.

See merge request !25