Update to Bevy 0.16.0-rc.1

[Jondolf]

Objective

The first release candidate for Bevy 0.16 has been released! We should migrate our main branch to it for early adopters, and to benefit from the various new features it introduces.

Solution

Migrate to Bevy 0.16.0-rc.1. Most changes were largely mechanical or minor cleanup, but some larger changes were also required.

The most notable change is that 0.16 removed the HierarchyEvent, so the AncestorMarkerPlugin had to be reworked. It now uses only observers instead of also using some systems, which significantly simplifies code and makes things more robust and schedule-free.

I also replaced some HashSet<Entity> types with EntityHashSet and HashMap<Entity> types with EntityHashMap.

Follow-Up

This is only a bare-bones migration to get Avian working with the Bevy 0.16 RC. We should take advantage of the new features!

Some ideas:

• no_std support
• Change ColliderParent to a relationship-like immutable ColliderOf component driven by hooks
• Rework collider debug rendering to use retained gizmos
• Migrate examples to the improved spawning API

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Migration Guide

Avian now supports Bevy 0.16.

The AncestorMarkerPlugin no longer requires a schedule or system set.