Faster directChildren and pickDirectChildren

[bjosttveit]

Description

A few hot-spot optimizations mostly related to heap allocation:

• Iterating allChildren and directChildren from LayoutPage should be faster and use much less memory by storing these as arrays instead of having to iterate a Map. A consequence is that removing nodes is a little more cpu-intensive.
• Using pickDirectChildren with a restriction (selecting a single row) for RepeatingChildrenPlugin now just returns the list of node-ids for that row directly and no longer needs to allocate a new array and loop through all of the other rows.
• Reduce the number of array/object allocations in setRowExtras.
• Avoid reading the context twice in useCtx() to check if it is provided or not. Believe it or not, but this showed up for memory allocation when profiling, and its a very easy win.

Related Issue(s)

• closes #{issue number}

Verification/QA

• Manual functionality testing
  • [x] I have tested these changes manually
  • [ ] Creator of the original issue (or service owner) has been contacted for manual testing (or will be contacted when released in alpha)
  • [ ] No testing done/necessary
• Automated tests
  • [ ] Unit test(s) have been added/updated
  • [ ] Cypress E2E test(s) have been added/updated
  • [x] No automatic tests are needed here (no functional changes/additions)
  • [ ] I want someone to help me make some tests
• UU/WCAG (follow these guidelines until we have our own)
  • [ ] I have tested with a screen reader/keyboard navigation/automated wcag validator
  • [x] No testing done/necessary (no DOM/visual changes)
  • [ ] I want someone to help me perform accessibility testing
• User documentation @ altinn-studio-docs
  • [ ] Has been added/updated
  • [x] No functionality has been changed/added, so no documentation is needed
  • [ ] I will do that later/have created an issue
• Support in Altinn Studio
  • [ ] Issue(s) created for support in Studio
  • [x] This change/feature does not require any changes to Altinn Studio
• Sprint board
  • [ ] The original issue (or this PR itself) has been added to the Team Apps project and to the current sprint board
  • [ ] I don't have permissions to do that, please help me out
• Labels
  • [x] I have added a kind/* label to this PR for proper release notes grouping
  • [ ] I don't have permissions to add labels, please help me out

[sonarcloud[bot]]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.2% Coverage on New Code
0.0% Duplication on New Code

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