[M1] add util::Error, Result<T>, format()

flake.nix

@@ -11,6 +11,11 @@
       let
         pkgs = import nixpkgs { inherit system; };
         llvmPkgs = pkgs.llvmPackages;
+        libcxxPkgs = {
+          catch2_3 = pkgs.catch2_3.override {
+            stdenv = llvmPkgs.libcxxStdenv;
+          };
+        };
       in {
         devShells.default = llvmPkgs.libcxxStdenv.mkDerivation {
           name = "cargoxx-dev";
@@ -25,6 +30,7 @@
           buildInputs = [
             pkgs.sqlite
             pkgs.reproc
+            libcxxPkgs.catch2_3
           ];
           env.NIX_CFLAGS_COMPILE = toString [
             "-stdlib=libc++"