add shell.nix and .envrc for development under nix

.envrc

@@ -0,0 +1 @@
+eval "$(lorri direnv)"

shell.nix

@@ -0,0 +1,32 @@
+with import <nixpkgs> {};
+let
+  src = fetchFromGitHub {
+      owner = "mozilla";
+      repo = "nixpkgs-mozilla";
+      # commit from: 2019-05-15
+      rev = "c482bfd3dab1bde9590b03e712d73ced15385be4";
+      sha256 = "18sxl0fxhbdnrfmkbmgxwsn12qy8dbv6ccb3imyyqbjqb76j8dpi";
+   };
+in
+with import "${src.out}/rust-overlay.nix" pkgs pkgs;
+stdenv.mkDerivation {
+  name = "rust-env";
+  buildInputs = [
+    (rustChannelOf { rustToolchain = ./rust-toolchain; }).rust
+    # latest.rustChannels.nightly.rust
+    # latest.rustChannels.stable.rust
+
+    clang
+    gettext
+    nettle
+    pkgconfig
+  ];
+
+  # Set Environment Variables
+  RUST_BACKTRACE = 1;
+
+  # compilation of -sys packages requires manually setting this :(
+  shellHook = ''
+    export LIBCLANG_PATH="${pkgs.llvmPackages.libclang}/lib";
+  '';
+}