build: provide Nix env and envrc for MSRV

.envrc

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

.gitignore

@@ -1,2 +1,3 @@
 /target
 Cargo.lock
+.direnv

shell.nix

@@ -0,0 +1,14 @@
+let
+  rust_overlay = import (builtins.fetchTarball "https://github.com/oxalica/rust-overlay/archive/master.tar.gz");
+  pkgs = import <nixpkgs> { overlays = [ rust_overlay ]; };
+  rustVersion = "1.56.1";
+  rust = pkgs.rust-bin.stable.${rustVersion}.default;
+in
+pkgs.mkShell {
+  buildInputs = [
+    rust
+  ] ++ (with pkgs; [
+    pkg-config
+  ]);
+  RUST_BACKTRACE = 1;
+}