Performance tuning - eager rune array conversion

> wc -l /tmp/list2
     2594098 /tmp/list2

    > time cat /tmp/list2 | fzf-0.10.1-darwin_amd64 -fqwerty > /dev/null

    real    0m5.418s
    user    0m10.990s
    sys     0m1.302s

    > time cat /tmp/list2 | fzf-head -fqwerty > /dev/null

    real    0m4.862s
    user    0m6.619s
    sys     0m0.982s

src/merger_test.go

@@ -22,7 +22,7 @@ func randItem() *Item {
 		offsets[idx] = Offset{sidx, eidx}
 	}
 	return &Item{
-		text:    &str,
+		text:    []rune(str),
 		index:   rand.Uint32(),
 		offsets: offsets}
 }