Add helper functions for multi-user toggle support

Introduced a new file `lib.nix` containing helper functions to streamline user package management and attributes for multi-user configurations. Updated various modules to utilize these functions, enhancing code maintainability and readability.

modules/dev/rust.nix

@@ -33,10 +33,8 @@ in
   config = lib.mkMerge [
     (lib.mkIf config.my.dev.rust.enable {
       users.users = let
-        users = if lib.isString config.my.toggleUsers.dev then [ config.my.toggleUsers.dev ] else config.my.toggleUsers.dev;
-      in lib.mkMerge (map (user: {
-        ${user} = { inherit packages; };
-      }) users);
+        userLib = import ../lib.nix { inherit lib; };
+      in userLib.mkUserAttrs config.my.toggleUsers.dev { inherit packages; };
     })
     {
       environment.variables.CARGO_HOME = "\${XDG_DATA_HOME}/cargo";