Add Nix development environment for the Nix users out there (#9)

This PR adds a Nix development environment that will allow Nix users to
easily build and work on valence without having to write their own.

If using flakes you can get into the development shell using `nix
develop`, otherwise you can use `nix-shell`

flake.nix

@@ -0,0 +1,31 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/master";
+    flake-utils.url = "github:numtide/flake-utils";
+    rust-overlay.url = "github:oxalica/rust-overlay";
+    flake-compat = {
+      url = "github:edolstra/flake-compat";
+      flake = false;
+    };
+  };
+
+  outputs = { self, nixpkgs, flake-utils, rust-overlay, ... }:
+  flake-utils.lib.eachSystem
+    [ "x86_64-linux" ]
+    (system:
+    let
+      overlays = [ (import rust-overlay) ];
+      pkgs = import nixpkgs {
+        inherit system overlays;
+      };
+    in 
+    rec
+    {
+      devShell = pkgs.mkShell {
+        buildInputs = with pkgs; [
+          (rust-bin.selectLatestNightlyWith (toolchain: toolchain.default))
+          rust-analyzer
+        ];
+      };
+    });
+}

shell.nix

@@ -0,0 +1,9 @@
+(import (
+  let
+    lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  in fetchTarball {
+    url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+    sha256 = lock.nodes.flake-compat.locked.narHash; }
+) {
+  src =  ./.;
+}).shellNix