Introduce Command::apply_raw

[james7132]

Objective

Partially address #20420. Command application currently requires a ptr::read_unaligned call ,which reads a complete copy of the command onto the stack before applying it to the World. This require quite a bit of extra memory read and written when the bundle or resource is otherwise very large, particularly if that memory is just going to be moved into the World anyway.

Solution

Add a new function to Command called apply_raw that takes the unaligned pointer provided by CommandQueue. Provide a default implementation for it using Command::apply. Rework (Raw)CommandQueue::push to work in terms of apply_raw.

This is an optional optimization and does not always need to be implemented by all Command types, just those likely to move a large amount of memory around.

An example of how to evade this extra copy was implemented for insert_resource, where the relevant metadata was pulled via read_unaligned, but the unaligned resource stored in the command was directly fed to World::insert_resource_by_id to skip the stack copy.

This unfortunately means we cannot use closure based Commands for these command types, but thankfully the number of performance sensitive commands like these is fairly low, and the concrete types do not need to be exposed as a public part of the interface.

TODO:

• Implement this extension for EntityCommands, specifically EntityCommands::insert and friends.
• Benchmark

Testing

Ran bevy_ecs tests and miri.

Future Considerations

• See if we can get around repeated CommandQueue reallocations without keeping permanently large command buffers around by using bumpalo or implementing our own bump allocation strategy within CommandQueue.

[james7132]

I went ahead and tried piping this through (Dynamic)Bundle. There's some UB with how it affects insertion of relationships, will need to investigate.