Attribute copying often uses identical logic for copying selected
elements or copying with an index map. Instead of reimplementing
this in each file, use the common implementation in the array_utils
namespace. This makes the commonality more obvious, gives improved
performance (this implementation is multithreaded), reduces binary
size (I observed a 173KB reduction), and probably reduces compile time.
This makes instance handling more consistent with all the other geometry
component types. For example, `MeshComponent` contains a `Mesh *` and
now `InstancesComponent` has a `Instances *`.
Differential Revision: https://developer.blender.org/D16137
