TODO pack decoding + pack exploding

• parallel
  • limit parallelism by allowing to set the amount of threads
• pack
  • extract borrowed objects from a pack
  • support streamed objects (similar to how it's done with loose objects) - no need, all slices support io::Read, and we don't actually support streaming, so let's net unify 'interfaces' on a low level like this.
• borrowed objects
  • write loose
    • blob
    • tag
    • tree
    • commit
• plumbing - explode pack
  • single threaded
  • multi-threaded
  • progress
  • statistics
• pack verify
  • add '--some-flag' to run every non-blob through a decode/encode cycle to see if all objects can be parsed
  • add that to the stress test
• multi-db (incorporate object lookup for loose objects and packs)
  • single threaded
  • optional object cache
  • fs-check - verify all object content of a git repository