Add TorrentStats.PiecesComplete

Fixes https://github.com/anacrolix/torrent/issues/352.

torrent.go

@@ -1760,6 +1760,7 @@ func (t *Torrent) statsLocked() (ret TorrentStats) {
 		}
 	}
 	ret.ConnStats = t.stats.Copy()
+	ret.PiecesComplete = t.numPiecesCompleted()
 	return
 }
 

torrent_stats.go

@@ -13,4 +13,5 @@ type TorrentStats struct {
 	ActivePeers      int
 	ConnectedSeeders int
 	HalfOpenPeers    int
+	PiecesComplete   int
 }