Add Nix development environment

The environment is impure, because it creates a Python venv in the local
directory. But this makes live easier with `pyo3-pack develop`.

Author: danieldk

.envrc

@@ -0,0 +1 @@
+use nix

default.nix

@@ -0,0 +1,37 @@
+with import <nixpkgs> {};
+let
+  mozillaOverlay = fetchFromGitHub {
+    owner = "mozilla";
+    repo = "nixpkgs-mozilla";
+    rev = "9f35c4b09fd44a77227e79ff0c1b4b6a69dff533";
+    sha256 = "18h0nvh55b5an4gmlgfbvwbyqj91bklf1zymis6lbdh75571qaz0";
+  };
+  rustNightly =
+    with import "${mozillaOverlay.out}/rust-overlay.nix" pkgs pkgs;
+    (rustChannelOf { date = "2019-02-07"; channel = "nightly"; }).rust;
+in stdenv.mkDerivation rec {
+  name = "toponn-env";
+  env = buildEnv { name = name; paths = buildInputs; };
+
+  nativeBuildInputs = [
+    pkgconfig
+    pyo3-pack
+    python3
+    rustNightly
+  ];
+
+  propagatedBuildInputs = [
+    python3Packages.numpy
+  ];
+
+  buildInputs = [
+    curl
+    libtensorflow
+    openssl
+  ] ++ lib.optional stdenv.isDarwin darwin.apple_sdk.frameworks.Security;
+
+  shellHook = ''
+    ${python3}/bin/python -m venv venv
+    source venv/bin/activate
+  '';
+}