Add an index argument to parallel iteration helpers in bevy_tasks

[msvbg]

Objective

bevy_tasks provides utilities for parallel mapping over slices. It can be useful to have a chunk index available in the iteration function to know which part of the original slice is being processed.

Solution

Adds an index argument to the parallel map functions in bevy_tasks.

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Changelog

Changed

• par_chunk_map, par_splat_map, par_chunk_map_mut, and par_splat_map_mut now provide a chunk index during iteration.

Migration Guide

Functions passed as arguments to par_chunk_map, par_splat_map, par_chunk_map_mut, and par_splat_map_mut must now take an additional index argument.

[msvbg]

Prior art:

• Rayon's Minimal Parallel Infrastructure With Bevy Task #11492
• Use rayon as the bevy_task thread pool #11995

[msvbg]

@alice-i-cecile Can you add a C-Breaking-Change to this one? (Or give me permission to add it myself)

[alice-i-cecile]

@msvbg invite to the Bevy Org sent, which will give you triage permissions :)