Keep an eye out for better doctest approaches

[georgefst]

In #107, we got rid of our custom setup, and tests are now run via cabal doctest. We would prefer something that worked with cabal test, as well as Stack, Nixpkgs and Haskell.Nix's native testing approaches.

It's likely that the solution will involve the new "setup hooks" (haskell/cabal#9551).

See #82 and #129 for more context.