[p_unification] I, for one, welcome our new nix overlords (#91)

Why
===

This repository hadn't been fully nixified.

What changed
============

This repository is now fully nixified.

Test plan
=========

```
direnv allow && poetry shell

python -m replit_river.codegen
```

no longer crashes due to a lack of stlibc++.so.

Author: lhchavez

.envrc

@@ -0,0 +1,3 @@
+use flake
+dotenv_if_exists
+source_env_if_exists .local.envrc

.gitignore

@@ -175,4 +175,7 @@ cython_debug/
 #.idea/
 
 # Vscode
-.vscode/
+.vscode/
+
+# nix
+.direnv/

flake.lock

@@ -0,0 +1,27 @@
+{
+  description = "A flake that provides tools needed to hack on river-python";
+
+  inputs.nixpkgs.url = "github:nixos/nixpkgs";
+
+  outputs = { self, nixpkgs }: let
+    mkPkgs = system: import nixpkgs {
+      inherit system;
+    };
+    mkDevShell = system:
+    let
+      pkgs = mkPkgs system;
+      replitNixDeps = (import ./replit.nix { pkgs = pkgs; }).deps;
+    in
+    pkgs.mkShell {
+      env = {
+        # Needed for Python/gRPC to be able to interact with libstdc++.
+        LD_LIBRARY_PATH = "${pkgs.stdenv.cc.cc.lib}/lib";
+      };
+      packages = replitNixDeps;
+    };
+  in
+  {
+    devShells.aarch64-darwin.default = mkDevShell "aarch64-darwin";
+    devShells.x86_64-linux.default = mkDevShell "x86_64-linux";
+  };
+}