BEP 46 Support: Updating Torrents Via DHT Mutable Items

This BEP concerns looking up torrents in the DHT via public key, instead of hash. This allows the original publisher to update by creating a new version, signing that version, and publishing it to the DHT under their key.

There isn't much for Intermodal to do here now, but it does include an extension to magnet links, which Intermodal should support:

```
magnet:?xs=urn:btpk:[ Public Key (Hex) ]&s=[ Salt (Hex) ]
```