Set up CI

[soenkehahn]

We used to have CI based on travis, but that is now defunct. We should have CI. Ideally we would test:

• multiple ghc versions,
• multiple versions of the dependencies.

It seems that neither stack nor haskell/actions support ghc before 7.10. Should we drop support for those old ghc versions?

[bergmark]

i think https://github.com/haskell-CI/haskell-ci supports quite old ghcs. but i don't think those old ghcs are important to support

[bergmark]

@soenkehahn and thanks for keeping this library alive! it's simple and i use it all the time. LMK if you want any help with it.

[soenkehahn]

@bergmark: Thanks a lot for your comments, very helpful! I've open #22 to set up CI, reviews appreciated!