Add nix flake (#97)

.gitignore

@@ -1,6 +1,6 @@
 /target
 **/*.rs.bk
-Cargo.lock
 /scratch
 docs/book
-.vscode/
+.vscode/
+/result

flake.nix

@@ -0,0 +1,52 @@
+{
+  description = "Steel";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable"; # We want to use packages from the binary cache
+    flake-utils.url = "github:numtide/flake-utils";
+    gitignore = {
+      url = "github:hercules-ci/gitignore.nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = {
+    self,
+    nixpkgs,
+    flake-utils,
+    gitignore,
+  }:
+    flake-utils.lib.eachDefaultSystem (system: let
+      inherit (gitignore.lib) gitignoreSource;
+      pkgs = nixpkgs.legacyPackages.${system};
+
+      manifest = pkgs.lib.importTOML ./Cargo.toml;
+      steel = with pkgs;
+        rustPlatform.buildRustPackage rec {
+          pname = manifest.package.name;
+          version = manifest.workspace.package.version;
+          src = gitignoreSource ./.;
+          cargoLock.lockFile = ./Cargo.lock;
+          buildInputs = [openssl] ++ lib.optionals stdenv.isDarwin [darwin.apple_sdk.frameworks.Security];
+          nativeBuildInputs = [
+            pkg-config
+          ];
+          # Test failing
+          doCheck = false;
+          postInstall = ''
+            mkdir $out/lib
+            export STEEL_HOME="$out/lib"
+            pushd cogs
+            $out/bin/steel install.scm
+            popd
+          '';
+        };
+    in rec {
+      packages.steel = steel;
+      legacyPackages = packages;
+      defaultPackage = packages.steel;
+      devShell = pkgs.mkShell {
+        buildInputs = with pkgs; [cargo];
+      };
+    });
+}