SORT exchange operator

[GLS at SU-AI (Guy Steele)]

Well, the use of such an operator would make the sort
somewhat slower, but I suppose it could be special-cased
the same way some predicates like ALPHALESSP are.
One can sort several arrays in parallel by first consing
together corresponding elements into single objects in
another array, sorting that, then decomposing and
storing back (yech).
Maybe SORT should just be a clever dispatch to other
user-available functions called MERGESORT, QUICKSORT, etc.

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