Writes the completed pieces of torrent to a file in directory.

If the torrent describes a single file, then that file with be created in directory with the torrent’s :name. If torrent describes multiple files, then a directory with the torrent’s :name will be created in directory, and the torrent’s files will be written to that directory.

The torrent will have its in-memory pieces cleared, and will be updated to remember the pieces that were written.