flake: add devShells

exposed via nix registry :D
can be used in nix-direnv like `use flake shells#cs.osu`

shells/rust/default.nix

@@ -0,0 +1,25 @@
+{
+  inputs,
+  mkShell,
+
+  pkg-config
+}:
+let
+  overlays = [ (import inputs.rust-overlay) ];
+  pkgs = import inputs.nixpkgs {
+    inherit system overlays;
+  };
+
+  toolchain = pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain;
+
+  rustPlatform = pkgs.makeRustPlatform {
+    inherit (toolchain) cargo rustc;
+  };
+in mkShell {
+  nativeBuildInputs = [
+    toolchain
+    pkg-config
+  ];
+
+  RUST_SRC_PATH = "${pkgs.rust.packages.stable.rustPlatform.rustLibSrc}";
+}