Speed up getting SharedArray items out

[mstoykov]

What?

Why?

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make ci-like-lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

[codecov-commenter]

Codecov Report

Merging #3245 (e02c32d) into master (447b159) will decrease coverage by 0.08%.
The diff coverage is n/a.

❗ Current head e02c32d differs from pull request most recent head 76e37db. Consider uploading reports for the commit 76e37db to get more accurate results

@@            Coverage Diff             @@
##           master    #3245      +/-   ##
==========================================
- Coverage   73.23%   73.15%   -0.08%     
==========================================
  Files         259      257       -2     
  Lines       19899    19894       -5     
==========================================
- Hits        14573    14554      -19     
- Misses       4402     4413      +11     
- Partials      924      927       +3     

Flag	Coverage Δ
ubuntu	73.15% <ø> (+0.01%)	⬆️
windows	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
js/modules/k6/data/share.go	46.15% <ø> (-7.70%)	⬇️

... and 10 files with indirect coverage changes