[m-rev.] diffs: speed up set_tree234.list_to_set

[Zoltan Somogyi]

I noted that converting the list of valid pred ids to a set in switch_detection.m
took up much larger percentage of that module's time than I thought reasonable.
These diffs speed up list_to_set, and the merge sort that it now (indirectly) calls.

Zoltan.
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