Replace ChildBufferCache with a simpler stack allocator.

[ickshonpe]

Objective

Replace the ChildBufferCache used in ui_stack_system with a simpler stack allocator.

Solution

Instead of the vec of vecs, use a single vec stack allocator.

Testing

The stack module's tests all still pass.

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Showcase

cargo run --example many_buttons --release --features="trace_tracy"

[PPakalns]

Using cnt, pop(), sort_by_key Reverse in some special cases could use even less memory, but probably in average case is just more complex code.

LGTM.

[PPakalns]

Using cnt, pop(), sort_by_key Reverse in some special cases could use even less memory, but probably in average case is just more complex code.

LGTM.

Just an optional idea:

One more optimization to avoid storing start, end on stack.

This function can be implemented as one while loop which at the start stores initial node_entity in child_buffer and then uses while let Some((entity, _)) = child_buffer.pop() to process nodes. When childrens are added, we use sort_by_key with Reverse wrapper helper type.

This would reduce amount of data stored on stack during recursion. But additional profiling is needed, maybe compiler already does some optimizations.

Recursive implementation of course is easier to understand than loop one.