stablerange: rearrange the code picking subrange to warm Same logic, as the previous changesets, we prepare the code before adding merge support.

stablerange: rearrange the reusing logic to prepare to merge We'll soon be able to reuse some lower range when dealing with merge too. So we prepare the code for this in advance for clarity.

merge-slicing: explain an alternative implementation in a comments It has a better time complexity so a C implementation would likely out perform the current implementation