This PR implements a merge sort algorithm on arrays. It has been
measured to be about twice as fast as `List.mergeSort` for large arrays
with random elements, but for small or almost sorted ones, the list
implementation is faster. Compared to `Array.qsort`, it is stable and
has O(n log n) worst-case cost. Note: There is still a lot of potential
for optimization. The current implementation allocates O(n log n)
arrays, one per recursive call.
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
