Add direnv and better support for Darwin

.envrc

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# the shebang is ignored, but nice for editors
+
+if type -P lorri &>/dev/null; then
+  eval "$(lorri direnv)"
+else
+  echo 'while direnv evaluated .envrc, could not find the command "lorri" [https://github.com/nix-community/lorri]'
+  use nix
+fi

shell.nix

@@ -0,0 +1,8 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.rustc
+    pkgs.cargo
+  ] ++ [ pkgs.libiconv pkgs.darwin.apple_sdk.frameworks.CoreServices ];
+}