Investigate usage of BEP 35.

I've been examining `.torrent` files for clues as to which features are widely used and supported, and to learn more about non-standard metadata extensions.

So far, I've looked at [this collection of torrents from 2014](https://archive.org/details/2014_torrent_archive_organized), which has been most elucidating.

Recently, I read [BEP 35](http://bittorrent.org/beps/bep_0035.html), which describes signed torrents. Signed torrents includes a `signatures` metainfo key, containing signatures and information about their signatories.

BEP 35 was released in 2016, but even so, the 2014 archive contains 13 torrents with signatures, so it must have been implemented or experimented with earlier.

I'm very interested in signed torrents, so I'd like to investigate use of BEP 35 further.

- [ ] find a collection of torrents more recent than 2014
- [ ] find which implementations BEP 35 exist
- [ ] figure out who, if anyone, created BEP 35 torrents
- [ ] find out why the standard didn't catch on
- [ ] BEP 35 seems to omit public keys if the signer doesn't have a non-root certificate. Why is this? How could a lone signature without a public key be valuable? (Perhaps the public key can be obtained from the signature.)