This implements RFC 136. This issue is agreed-upon by the @nixos/nix-team

• Rename nix nar dump-path to nix nar pack (reserving unpack for the reverse operation)
• Possible extensions for follow-ups:
  • Make sure the command uses all global flags (currently it doesn't)
  • add unpack to create a local directory
  • Read files to cat and ls from pipe, take arbitrary many on the command line (might get tricky due to sequetiality of reads though)
  • Add direct conversion from and to tar