merge(3p/immer): Subtree merge at 'ad3e3556d' as 'third_party/immer'

Change-Id: I9636a41ad44b4218293833fd3e9456d9b07c731b

third_party/immer/default.nix

@@ -0,0 +1,14 @@
+with import <nixpkgs> {};
+
+stdenv.mkDerivation rec {
+  name = "immer-git";
+  version = "git";
+  src = fetchGit ./.;
+  nativeBuildInputs = [ cmake ];
+  dontBuild = true;
+  meta = with stdenv.lib; {
+    homepage    = "https://github.com/arximboldi/immer";
+    description = "Postmodern immutable data structures for C++";
+    license     = licenses.boost;
+  };
+}