Add new functions to `array_utils` namespace called `gather(..)`. Versions of `GVArray::materialize_compressed_to_uninitialized(..)` with threading have been reimplemented locally in multiple geometry node contexts. The purpose of this patch is therefore to: * Assemble these implementations in a single file. * Provide a naming convention that is easier to recognize. Differential Revision: https://developer.blender.org/D15786