Add a nix-based devshell

[voidus]

I've added this for myself and thought maybe you're interested in this as well.

Obviously, if not, feel free to just close this :)

[edsko]

This library doesn't have any system deps so I'd prefer not to add any nix stuff; cabal should be able to handle the dependencies just fine? If there is a good reason for why nix is necessary here, let me know :)

[voidus]

All good, I personally got rid of my global dev setup completely :)