[WIP, RFC] torrent file parser

This is a work in progress, I'm creating this pull request to get some reviews/comments.

Things to do:

- [x] Implement `toBEncode` methods.
- [x] Expose lenses.
- [x] Implement `Arbitrary` instances.
- [x] Add tests. (some example files files https://github.com/Fuuzetsu/torrentinfo/tree/master/test/files)

How should I add tests? Should I add some example torrent files and try to parse them? As for properties, all I can think is `fromBEncode . toBEncode $ d = Right d`. Any other ideas?
I would suggest dropping the distinction between single-file and multi-file torrents. The only time the distinction is important is when parsing (or serialising, I guess) the torrent file. Afterwards, single-file torrents are just an uninteresting special case of multi-file torrents, so separating the two just adds some complexity without buying us anything.