add shell.nix and nix-community's flake-compat

- `shell.nix` allows running the package with `nix-shell --run`
- taking the version from `flake.lock` is more explicit and uniform

default.nix

@@ -1,11 +1,14 @@
-# This file is the compt layer of flakes: https://github.com/edolstra/flake-compat
-# See flake.nix for details
-(import (
-  let
-    lock = builtins.fromJSON (builtins.readFile ./flake.lock);
-  in fetchTarball {
-    url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
-    sha256 = lock.nodes.flake-compat.locked.narHash; }
-) {
-  src =  ./.;
-}).defaultNix
+{ system ? builtins.currentSystem }:
+let
+  lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+
+  inherit (lock.nodes.flake-compat.locked) owner repo rev narHash;
+
+  flake-compat = fetchTarball {
+    url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
+    sha256 = narHash;
+  };
+
+  flake = import flake-compat { inherit system; src = ./.; };
+in
+flake.defaultNix

flake.nix

@@ -3,10 +3,7 @@
 
   inputs = {
     nixpkgs.url = "nixpkgs/nixos-unstable";
-    flake-compat = {
-      url = "github:edolstra/flake-compat";
-      flake = false;
-    };
+    flake-compat.url = "github:nix-community/flake-compat";
   };
 
   outputs = { self, nixpkgs, flake-compat }:

shell.nix

@@ -0,0 +1,14 @@
+{ system ? builtins.currentSystem }:
+let
+  lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+
+  inherit (lock.nodes.flake-compat.locked) owner repo rev narHash;
+
+  flake-compat = fetchTarball {
+    url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
+    sha256 = narHash;
+  };
+
+  flake = import flake-compat { inherit system; src = ./.; };
+in
+flake.shellNix