Introduce new Chunk iteration APIs

[teh-cmc]

• Way more ergonomic
• Same performance
• Less code
• Support for structs!
• Basis for new upcoming codegen'd deserialization APIs (...at some point)

[github-actions]

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Result	Commit	Link	Manifest
✅	c77bd2a	https://rerun.io/viewer/pr/8553	+nightly +main

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