Simplify Item structure

This commit compensates for the performance overhead from the
extended tiebreak option.

src/merger_test.go

@@ -23,7 +23,7 @@ func randItem() *Item {
 	}
 	return &Item{
 		text:    []rune(str),
-		index:   rand.Int31(),
+		rank:    buildEmptyRank(rand.Int31()),
 		offsets: offsets}
 }