Add support for multiple files and dictionaries (BitTorrent v1)

[marcinczenko]

This will be twofold:

1. Add standard support for multiple files and dictionaries (this is will complete the support for BitTorrent v1)
2. On top of that investigate supporting downloading a single file from the published folder (possible this item will become its own item).

This issue became something bigger than just adding support for files and folders to BitTorrent. Actually at the same time we are trying to research and try supporting files and folders in Codex in a way that the files and dirs can be individually addressable. On top of that I am looking into supporting mutations, trying to get some inspirations from PerKeep and Btrfs file system from Fedora.

[marcinczenko]

Some initial work has been done: codex-storage/nim-codex#1202 + uploading and downloading directories.