Chunks are merged in reverse

[Yomguithereal]

Hello @dapper91,

I think here the index of the chunk should also be reversed to avoid the max-heap reversing the chunk in the output, which mean the output sort order is not stable.

Ref: medialab/xan#619

[dapper91]

@Yomguithereal Hi,

Thanks for the report. Fixed the bug in release 0.1.5.

[Yomguithereal]

Thanks for the fix @dapper91. Closing the issue now :)