* Point to more recent nixpkgs commit. This fixes build issue with python
3.7.3 (only on darwin?).
* Separate shell.nix and default.nix. This is so that lorri can be used for
development.
* Add nix output to .gitignore.
* Format *.nix using nixfmt.
* Use gitignore for filtering files in nix derivation.
It was decided that we're going to make the project compatible with just one
GHC version at a time. Right now this version is going to be 8.6.4.
A small refactoring included, plus support for the “deriving via” feature.
