This is in preparation for eventual hierarchical bone collections.
The motivation here is that this will allow us to efficiently specify
children as an index range, which would be inefficient with a listbase
due to the list traversal overhead incurred for index-based look ups.
We're still saving to blend files as a list base for forwards compatibility
with Blender 4.0, but storing as an array at runtime for efficient indexing.
This should not result in any user-visible changes.
Pull Request: https://projects.blender.org/blender/blender/pulls/115354
9e7a70d6c6