Update code to run with ghc-8.10.7

[henrylaxen]

Hi Robert,
I was looking around for examples of Free Monads and stumbled across this repository. When I tried to compile it, I got quite a few compiler errors, mostly relating to MonadFail. Anyway I fixed them and got the code to compile. I tried (many times) to push my new code to a fork of your repository and always got:

remote: Permission to henrylaxen/GraphPlay.git denied to henrylaxen.
fatal: unable to access 'https://github.com/henrylaxen/GraphPlay.git/': The requested URL returned error: 403

This happened even after a generated a new token. I really don't know what is going on.
So I gave up, created a tgz file, and put it here
If you are interested, you can just download it. The files I changed were:

  "graphplay.cabal"
  "play/Deprecated/DiGraph/TreeMonoidFoldOnSimpleGraph.hs"
  "play/Play.hs"
  "play/S1_Cstr/E04_PolyToMax.lhs"
  "play/S2_Free/E03_BFS2.lhs"
  "src/FreeDSL/VWalk.hs"
  "src/FreeDSL/VWalk/InterpreterFold.hs"
  "src/FreeDSL/VWalk/InterpreterStd.hs"
  "src/PolyGraph/Common/RecursionHelpers.hs"
  "src/PolyGraph/ReadOnly/DiGraph/Fold/TAFold.hs"
  "src/PolyGraph/ReadOnly/DiGraph/Fold/TAMonoidFold.hs")

I hope this is helpful.
Best wishes,
Henry Laxen