feat(assign1): add basic "Hello World" program

flake.nix

@@ -7,11 +7,21 @@
   };
 
   outputs = { self, nixpkgs, systems, ... }:
-    let eachSystem = nixpkgs.lib.genAttrs (import systems);
+    let
+      subdirs = [
+        "assign1"
+      ];
+
+      eachSystem = nixpkgs.lib.genAttrs (import systems);
     in {
       packages = eachSystem (system:
         let pkgs = nixpkgs.legacyPackages.${system};
-        in {});
+        in pkgs.lib.mergeAttrsList
+          (builtins.map (d:
+            builtins.mapAttrs
+              (_: v: pkgs.callPackage v {})
+              (import ./${d}))
+            subdirs));
 
       devShells = eachSystem (system:
         let pkgs = nixpkgs.legacyPackages.${system};