greet.hs needs cleanup

We currently have a haskell module in `README.md` and another one in `example/greet.hs`. I think the two have diverged at some point in the past, but even if they've always been different, there should only be one example, so I suggest:

- [ ] replace the haskell code in `README.md` with a link to `example/greet.hs`
- [ ] make sure that `example/greet.hs` contains all the information from the README.
- [ ] add ci machinery that runs `greet.hs` and compares the output to `greet.md`, so the example can't get outdated any more.