Bump nixpkgs and simplify/clean up nix setup (#1529)

* Refactor tests and fix README

* fix linting hints from new hlint version

* postgrest-style with new nixpkgs-fmt

nix/style.nix

@@ -1,37 +0,0 @@
-{ writeShellScriptBin
-, git
-, nixpkgs-fmt
-, silver-searcher
-, stylish-haskell
-, buildEnv
-}:
-let
-  style =
-    writeShellScriptBin "postgrest-style"
-      ''
-        set -euo pipefail
-
-        rootdir="$(${git}/bin/git rev-parse --show-toplevel)"
-
-        # Format Nix files
-        ${nixpkgs-fmt}/bin/nixpkgs-fmt "$rootdir" > /dev/null 2> /dev/null
-
-        # Format Haskell files
-        ${silver-searcher}/bin/ag -l -g '\.l?hs$' "$rootdir" \
-          | xargs ${stylish-haskell}/bin/stylish-haskell -i
-      '';
-
-  check =
-    writeShellScriptBin "postgrest-style-check"
-      ''
-        set -euo pipefail
-
-        ${style}/bin/${style.name}
-
-        ${git}/bin/git diff-index --exit-code HEAD -- '*.hs' '*.lhs' '*.nix'
-      '';
-in
-buildEnv {
-  name = "postgrest-style";
-  paths = [ style check ];
-}