This branch showcases the invasive approach of using Stack+Nix integration.

$ nix-shell
[nix-shell]$ stack build

The problem with this approach is:

• Either we modify stack.yaml (this is what we did in this branch) to pass the various Nix options, but now non-Nix users can no longer build it
• Or we don't modify stack.yaml and Nix users must pass the various flags in the CLI every time, e.g.:

  $ nix-shell
  [nix-shell]$ stack --nix --no-nix-pure --nix-shell-file=nix/stack-integration.nix build