https://www.bittorrent.org/beps/bep_0003.html

This uses an index to quickly look up what pieces are part of a
file, and then checks that they are all valid, this should be faster
as it is likely that the torrent has more pieces than files (few
large files, vs many, many small files)..

This needs to be expanded a bit, and handle utf-8 encoded filenames..