nix: add shell.nix

This enables a build environment using nix. Should work on most linux
varieties but I've only tested on NixOS.

shell.nix

@@ -0,0 +1,8 @@
+{ pkgs ? import <nixpkgs> {} }:
+with pkgs;
+mkShell {
+  buildInputs = [ 
+    perl perlPackages.YAML perlPackages.TemplateToolkit
+    lmdb zstd secp256k1 libb2 flatbuffers zlib openssl libuv
+  ];
+}