Add contracts for `SmallSort` #1225

Job	Run time
Kani List	9m 45s
Verify std library (partition 1)	40m 19s
Verify std library (partition 2)	35m 2s
Verify std library (partition 3)	27m 27s
Verify std library (partition 4)	43m 22s
Verify std library (partition 1)	46m 38s
Verify std library (partition 2)	26m 14s
Verify std library (partition 3)	25m 42s
Verify std library (partition 4)	6h 0m 23s
	10h 14m 52s

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