Add ability to download multi-file torrents

TODO:

[ ] Sort out benchmarks? Currently doesn't seem to be very useful as it's not really testing real world scenario of using the iovecs abstraction (the tested against baseline is not simulated correctly as we're reusing iovecs + forcing a dynamic allocation).
[x] Finish directory integration tests: currently the test fails with the error that the seed is not in fact a seed. The directory is probably not picked up by transmission, so I need to investigate what is actually being seeded and what bitfield is being sent. The error may possibly lie in cratetorrent's parsing of metainfo, maybe we're not picking up the expected files and thus bitfield correctly.
[ ] More unit tests? Directory saving API tests in the top level disk module (though something similar already exists in disk::io). Also maybe related to the previous task: verifying that multi-file torrent metainfos are parsed correctly.

closes: #33 closes: #27 closes: #24 closes: #21 closes: #17

vimpunk / cratetorrent