build(deps): add rug for arbitrary precision math

flake.nix

@@ -14,16 +14,22 @@
 
         cargoToml = fromTOML (builtins.readFile ./Cargo.toml);
         name = cargoToml.package.name;
+
+        buildInputs = with pkgs; [
+          m4
+        ];
       in
       {
         packages.default = pkgs.rustPlatform.buildRustPackage {
-          inherit name;
+          inherit name buildInputs;
           src = ./.;
 
           cargoLock.lockFile = ./Cargo.lock;
         };
 
         devShells.default = pkgs.mkShell {
+          inherit buildInputs;
+
           packages = with pkgs; [
             rustc
             cargo