Refactor to simplify merging child slabs in array

[fxamacker]

Updates #464

Currently, ArrayMetaDataSlab.MergeOrRebalanceChildSlab() is complicated with over 300 lines of code.

This PR refactors merging in this function by adding new two helper functions mergeChildren() and updateChildrenHeadersAfterMerge().

This PR also uses go1.21 slices package to simplify slice operations.

---

• Targeted PR against main branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[turbolent]

Nice work refactoring into smaller and reusable functions!

I assume the code was extracted using automatic refactoring (e.g. IDE)? I didn't fully check if it matches exactly the old code (given the diff is hard to read)

[fxamacker]

I assume the code was extracted using automatic refactoring (e.g. IDE)? I didn't fully check if it matches exactly the old code (given the diff is hard to read)

@turbolent Yeah, the diff for this PR is hard to read.

The code was extracted with a combination of manual and automatic refactoring.

• I created new variables leftSlab and rightSlab, and assigned them to different slabs depending on the merge conditions. For example, if child merges with rightSib then leftSlab, rightSlab = child, rightSib.
• I extracted common functionality using IDE, followed by some manual adjustments.

I re-reviewed this PR and have been running brief smoke tests (on my desktop) for PRs with these types of changes and no problems were found.

After the remaining 10 PRs for issue #464 are merged I will:

• run longer smoke tests on gcp.
• look into including these changes in backwards compatibility test, etc.

Thanks again for these PR reviews and questions! 👍

[turbolent]

Thanks for the explanation and double checking! 👍