Performance improvements to ``np.unique`` for string dtypes
-----------------------------------------------------------
The hash-based algorithm for unique extraction provides
an order-of-magnitude speedup on large string arrays. 
In an internal benchmark with about 1 billion string elements,
the hash-based np.unique completed in roughly 33.5 seconds,
compared to 498 seconds with the sort-based method
– about 15× faster for unsorted unique operations on strings.
This improvement greatly reduces the time to find unique values
in very large string datasets.