Add nix/flake support

Example:

nix build github:mimblewimble/grin-wallet

./result/bin/grin-wallet --help

.gitignore

@@ -11,3 +11,4 @@ grin.log
 wallet.seed
 test_output
 .idea/
+result

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1714024283,
+        "narHash": "sha256-5tiVotLS0l3U8PyGMzI6tRdepcep2DlBDKTV7zYR/vE=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "6abeba70778c0e40f64910956d957025deac2f34",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "release-23.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,49 @@
+{
+  description = "Reference Grin Wallet.";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/release-23.11";
+  };
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      forAllSystems = with nixpkgs; lib.genAttrs lib.systems.flakeExposed;
+
+      nixpkgsFor = forAllSystems (
+        system:
+        import nixpkgs {
+          inherit system;
+          overlays = [ self.overlay ];
+        }
+      );
+    in
+    {
+      overlay =
+        final: prev: with final; {
+          grin-wallet = pkgs.rustPlatform.buildRustPackage {
+            pname = "grin-wallet";
+            version = "5.3.0";
+            src = ./.;
+
+            cargoLock = {
+              lockFile = ./Cargo.lock;
+            };
+
+            nativeBuildInputs = with pkgs; [
+              pkg-config
+              clang
+            ];
+            buildInputs = [ pkgs.openssl ];
+            LIBCLANG_PATH = "${pkgs.libclang.lib}/lib";
+
+            # do not let test results block the build process
+            doCheck = false;
+          };
+        };
+
+      packages = forAllSystems (system: {
+        default = nixpkgsFor.${system}.grin-wallet;
+      });
+    };
+}