Add nix and envrc setup.

.envrc

@@ -0,0 +1 @@
+use_nix

default.nix

@@ -0,0 +1,19 @@
+let
+  unstableTarball = fetchTarball
+    "https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz";
+  pkgs = import <nixpkgs> { };
+  unstable = import unstableTarball { };
+
+in with pkgs;
+pkgs.mkShell {
+  name = "rust";
+  buildInputs = [
+    openssl
+    pkg-config
+    unstable.cargo
+    unstable.rustc
+    unstable.rustfmt
+    unstable.rust-analyzer
+    postgresql
+  ];
+}