Use required components for component initialization

[Jondolf]

Objective

Bevy 0.15 got support for required components! We can replace a lot of our custom-made automatic component insertion with them, reducing the number of archetype moves and making startup more efficient.

Solution

Use required components for most component initialization logic.

I have also made CollidingEntities optional, as adding it automatically for every collider wastes memory unnecessarily. It must now be added manually.

Follow-ups

• Make Position and Rotation required and rework init_transforms (ran into some ordering issues, so left it to a follow-up)
• Make force and impulse components optional
• Make friction and restitution components optional, and default to globally defined default material properties

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Migration Guide

The CollidingEntities component is no longer added automatically. To read entities that are colliding with a given entity, you must now add the CollidingEntities component for it manually.

To revert to the old behavior, you can also make CollidingEntities a required component for colliders:

app.register_required_components::<Collider, CollidingEntities>();