Test suite build failure w/ ghc 8.8

[DanBurton]

        • Could not deduce (MonadFail m) arising from a use of ‘fail’
          from the context: Monad m
            bound by the type signature for:
                       readHex' :: forall (m :: * -> *). Monad m => String -> m [Word8]
            at src/Tests/HexUtils.hs:22:1-42