See https://developer.blender.org/D9499.
* Use `blender::Map` over `GHash`
* Use `blender::Vector` over allocated `ListBase *`
Benefits:
* Significantly reduces the amount of heap allocations in large trees (e.g.
from O(n) to O(log(n)), where n is number of objects).
* Higher type safety (no `void *`, virtually no casts).
* More optimized (e.g. small buffer optimization).
* More practicable, const-correct APIs with well-defined exception behavior.
Code generally becomes more readable (less lines of code, less boilerplate,
more logic-focused APIs because of greater language flexibility).
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