[5.4] Faster implementation for Arr::crossJoin()

[vlakoff]

Follow-up to #19236. ping @JosephSilber

Benchmarks, for 1000 iterations:

Arr::crossJoin([1, 2], ['a', 'b'], ['I', 'II', 'III']);

current: 429 ms
new: 30 ms

Arr::crossJoin([1, 2], ['a', 'b', 'c']);

current: 205 ms
new: 18 ms

props Serg for this implementation. (note I removed the array_filter line)

Also added a few tests to reinforce BC.

[vlakoff]

Changed tests to assertSame for strict array comparisons. This implementation's results have the same internal ordering as previous code.

[thecrypticace]

👍

Your reminder that building arrays with reduce, while fun, is accidentally quadratic.
https://twitter.com/AirspeedSwift/status/627469657264553984

(This is about Swift but the same concept applies here as well)

[vlakoff]

I just ran some quick'n'dirty tests, this new implementation seems to be O(n), and it seems the previous implementation was also O(n).

So the new implementation is just constantly faster than the previous one :)