Add default.nix & direnv.

.envrc

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

default.nix

@@ -0,0 +1,27 @@
+let
+  pkgs = import <nixpkgs> {
+    overlays = [
+      (import (builtins.fetchTarball
+        "https://github.com/oxalica/rust-overlay/archive/master.tar.gz"))
+    ];
+  };
+
+  rust = pkgs.rust-bin.stable.latest.rust.override {
+    extensions = [ "rust-src" ];
+  };
+
+in with pkgs;
+pkgs.mkShell rec {
+  name = "rust";
+  buildInputs = [
+    openssl
+    pkg-config
+    cargo
+    rust
+    rustfmt
+    rust-analyzer
+    wasm-pack
+    wasm-bindgen-cli
+    nodePackages.rollup
+  ];
+}