Expose bundling functions from default.nix

[takeda]

If one tries to create bundle directly from Nix expression it is much more convenient and cleaner to use these functions directly instead of going through the CLI command.

I currently do:

  nix-bundle = import (pkgs.nix-bundle + "/share/nix-bundle/default.nix") { nixpkgs = pkgs; };

But that feels hacky.