replace partial_cmp with total_cmp and use unstable sort for faster sorting

[Ahmad-Alsaleh]

This commit has three changes:

1. use total_cmp instead of partial_cmp to compare floats. Unlike patial_cmp, total_cmp doesn't return an error.
2. replace Vec::sort_by() with Vec::sort_unstable_by() to increase sorting speed.
3. avoid calling Vec::reverse() after sorting by comparing rank2 to rank1 instead of rank1 to rank2.