Convert to Nix flake
This setup allows me to:
- 
ditch Lorri one less implicit, global dependency 
- 
pin version of Nixpkgs So it's more reproducible than standard default.nix that depends on host environment. Now the revision is in flake.lock file. 
The downside is that at the time of this commit this requires experimental features of Nix that are not enabled by default. See https://nixos.wiki/wiki/Flakes
That's why I'm reluctant to remove default.nix yet. For compatibility with the current version of Nix I'm using flake-compat library.