Make uniqueItems fast

[ChALkeR]

Currently, uniqueItems on non-primitives is O(n^2) both here and in ajv.
It should be safe as complexityChecks catch unbounded uniqueItems.

But there is no reason for uniqueItems to be O(n^2), even when dealing with complex objects.

See implementation in https://github.com/ChALkeR/array-is-unique/blob/main/sorting.js -- that reaches O(n * log n).
That shouldn't exceed O(n * log n) even in worst-case on modern JS engines which use Timsort.

Also, sort-based implementation is faster than hash/set-based even on primitives, and has significantly lower memory consumption.

[ChALkeR]

That impl can be made faster in non-unique cases by stopping sorting of large arrays with a throw.

[ChALkeR]

Seems to be non-trivial due to certain limitations. Will revisit for 1.x