refactor: move rust dev shell into 'rust' directory

rust/flake.nix

@@ -0,0 +1,32 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+  };
+
+  outputs =
+    inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+
+      perSystem =
+        { pkgs, ... }:
+        {
+          devShells.default = pkgs.mkShell {
+            buildInputs = with pkgs; [
+              cargo
+              clippy
+              evcxr
+              rust-analyzer
+              rustc
+              rustfmt
+            ];
+          };
+        };
+    };
+}