`sort` with a region argument

[mppf]

PR #25636 adds a proc sort that accepts a region argument to indicate that only a portion of the array should be sorted. This is intended as a performance improvement because the main alternative of using array slices adds significant overhead for some sorting algorithms.

This issue tracks the TODO of making this variant of proc sort stable and serves as a place for discussion.

If you find yourself needing proc sort with a region argument, please add a comment here.