Turn apply_deferred into a ZST System

[ItsDoot]

Objective

• Required by Unify ExclusiveSystemParams with normal SystemParams #16622 due to differing implementations of System by FunctionSystem and ExclusiveFunctionSystem.
• Optimize the memory usage of instances of apply_deferred in system schedules.

Solution

By changing apply_deferred from being an ordinary system that ends up as an ExclusiveFunctionSystem, and instead into a ZST struct that implements System manually, we save ~320 bytes per instance of apply_deferred in any schedule.

Testing

• All current tests pass.

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Migration Guide

• If you were previously calling the special apply_deferred system via apply_deferred(world), don't.

[bushrat011899]

Nice! I'm in favour of a breaking change to rename this ApplyDeferred. Users aren't supposed to actually call this as a function anyway, and giving it a PascalCase name should help highlight to users that it isn't a normal system.

Memory saving is also nice, especially since this system will be reused a lot in a typical application.

[ItsDoot]

I've gone ahead and renamed apply_deferred to ApplyDeferred and added a deprecated alias for the old identifier. It's not a big lift on our end to include it (4 lines), so for the odd dev still using apply_deferred directly, this will give them an immediate actionable solution.