Add a `torrent from-link` subcommand

[atomgardner]

This is a first cut of the peer protocol and from-link subcommand.

~Would love if you could give it a quick review. It obviously needs more tests.~

~This has tests now. I have local patch that adds a --thread-per-peer flag, and it speeds things up like 100 fold, but it is extremely hacky.~

The command now uses rayon, and is acceptably quick. It'd be quicker if it pipelined peers from the tracker.

[atomgardner]

One more thing to do before merging:

• [x] tidy up the peer module tree