Fix stablePartition(subrange:by:) using subrange count instead of entire collection count.

[pmtao]

Function stablePartition(subrange:by:) in Partition should not using the collection count which will cause bound error, but using subrange count.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[kylemacomber]

Good catch!

[natecook1000]

Thanks for this fix, @pmtao! 👏👏👏