nix: add overlays, packages, apps

flake.nix

@@ -11,12 +11,45 @@
     , flake-utils
     , rust-overlay
     }:
+    let
+      inherit (builtins) fromTOML readFile substring;
+
+      cargoToml = fromTOML (readFile ./Cargo.toml);
+      version = "${cargoToml.package.version}+${substring 0 8 self.lastModifiedDate}_${self.shortRev or "dirty"}";
+
+      mkFimt = { lib, rustPlatform, ... }:
+        rustPlatform.buildRustPackage {
+          pname = cargoToml.package.name;
+          inherit version;
+
+          src = lib.cleanSource ./.;
+          cargoLock.lockFile = ./Cargo.lock;
+
+          doCheck = true;
+        };
+    in
+    {
+      overlays.fimt = final: prev: {
+        fimt = prev.callPackage mkFimt;
+      };
+      overlays.default = self.overlays.fimt;
+    }
+    //
     flake-utils.lib.eachDefaultSystem (system:
     let
       overlays = [ (import rust-overlay) ];
       pkgs = import nixpkgs { inherit system overlays; };
+      fimt = pkgs.callPackage mkFimt { };
     in
     {
+      packages = {
+        inherit fimt;
+        default = fimt;
+      };
+      apps = let fimtApp = flake-utils.lib.mkApp { drv = fimt; }; in {
+        fimt = fimtApp;
+        default = fimtApp;
+      };
       devShell = pkgs.mkShell {
         nativeBuildInputs = [ ];
         buildInputs = [