refactored nixos directory & updated README

nixos/rust.nix

@@ -0,0 +1,31 @@
+{ inputs, pkgs, ... }:
+
+{
+  # Apply the overlay to the package set
+  nixpkgs.overlays = [
+    inputs.rust-overlay.overlays.default
+  ];
+
+  environment.systemPackages = with pkgs; [
+    (rust-bin.fromRustupToolchainFile ./rust-toolchain.toml)
+    evcxr #rust repl
+    taplo #toml formatter & lsp
+    cargo-watch
+    cargo-deny
+    cargo-audit
+    cargo-update
+    cargo-edit
+    cargo-outdated
+    cargo-license
+    cargo-tarpaulin
+    cargo-cross
+    cargo-zigbuild
+    cargo-nextest
+    cargo-spellcheck
+    cargo-modules
+    cargo-bloat
+    cargo-unused-features
+    bacon
+    lldb_16
+  ];
+}