Don't panic in the ECS where we know a fetch is valid

[james7132]

Objective

Partly addresses #12107. We're using panicking Index/IndexMut fetches on Tables and Archetypes in potentially hot operations in the ECS. This creates a lot of extra codegen when we're we know the corresponding Table or Archetype exists.

Solution

• Remove the Index and IndexMut implementations on Tables and Archeytpes, and replace them with get/get_mut where appropriate.
• Create and use Tables::empty and Tables::empty_mut as the empty table is always guaranteed to exist.
• Use DebugCheckedUnwrap in areas where we we know the table or archetype exist.
• Create a UnsafeVecExtensions trait that provides a Vec::swap_remove_unchecked that is the same as swap_remove but without the safety check. Use it where we are using swap_remove like removing component ticks.
• Do the same with a number of areas where we're currently using unwrap.

TODO: Clean up the invariants on unsafe functions and their callsites.

Performance

This PR primarily impacts Entity{Ref/Mut/WorldMut}, command application, and direct World interaction.

TODO: Benchmark.

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Changelog

TODO

Migration Guide

TODO