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• add ci machinery that runs greet.hs and compares the output to greet.md, so the example can't get outdated any more.