Some notes on Nix language style

In an attempt to keep this more consistent than NixWRT ended up being, here is a Nix language style guide for this repo.

favour callPackage over raw import for calling derivations or any function that may generate one - any code that might need pkgs or parts of it.
prefer let inherit (quark) up down strange charm over with quark, in any context where the scope is more than a single expression or there is more than one reference to up, down etc. with pkgs; [ foo bar baz] is OK, with lib; stdenv.mkDerivation { ... } is usually not.
is defined only when running tests, so don't refer to it in "application" code
the parameters to a derivation are sorted alphabetically, except for lib, stdenv and maybe other non-package "special cases"
indentation is whatever emacs nix-mode says it is.
- where a let form defines multiple names, put a newline after the token let, and indent each name two characters
should it be a package or a module? packages are self-contained - they live in /nix/store/eeeeeee-name and don't directly change system behaviour by their presence or absense. modules can add to /etc or /bin or other global state, create services, all that side-effecty stuff. generally it should be a package unless it can't be.