Torrent serialization and tracker parsing

[angrynode]

No description provided.

[angrynode]

Tests are failing because of tests/libtorrent/good/sample.torrent, which contains a tracker tracker.publicbt.com:80/announce (without scheme).

Some notes:

• it appears in qBittorrent but unsupported URL protocol
• it doesn't appear at all in Transmission
• it appears in imdl

Two notes:

• my bad for assuming sample.torrent was actually a good example, it's actually a bad one (i guess?)
• maybe we can have relaxed scheme validation (eg. unknown scheme or no scheme) behind a feature flag? or not at all, but by default i believe this should fail to parse indeed

[angrynode]

added unknown_tracker_scheme crate feature which produces a new TrackerScheme::Unknown(String) variant.

There's tests for both cases.