[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?
It's not just about optimisation (even though it does tend to help avoiding space leaks), I also find it easier to reason about (no exceptions raised in unexpected places etc, although that's unlikely to be a problem here). But as I said, it's a matter of taste, feel free to do it differently.