[11.x] Collection average/avg optimization

[bert-w]

This PR includes an algorithm improvement for the Collection avg() and average() function.

The previous code used an inefficient map+filter+count, whereas the new implementation does it all at once with a reducer.

Algo explanation

1. Start with a reducer array with 2 empty elements [0, 0]
2. Loop/reduce the array:
   • add (sum) all non-null values to the first array element
   • increment the second array element with 1 (the count)
3. After loop: $avg = $reduced[0] / $reduced[1]

Additionally, the function has been moved to the EnumeratesValues trait so the lazy collection can also benefit from this optimization. It also works as expected with yield.

Benchmark (gist)

Size	Old (ms)	New (ms)	% Improvement
10	0.0707	0.0614	13.14%
100	0.3149	0.221	29.78%
1,000	3.1159	2.2254	28.57%
10,000	31.0282	23.3528	24.71%
100,000	315.3512	232.7599	26.22%
1,000,000	3,219.9169	2,151.9144	33.17%

[github-actions]

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