Build using the latest (3.6.3) release of husk-scheme

This pull request includes a couple of small changes to let flowskell build with husk-scheme 3.6.3. Basically, the final API for `evalLisp'` is:

    evalLisp' :: Env -> LispVal -> IO (ThrowsError LispVal)

This returns an `Either` type, so errors can be extracted directly using `Left`.

Thanks,

Justin