Change ResourceEntities from SparseSet to SparseArray to speed up resource lookups

[chescock]

Objective

Reduce memory usage for resources, and maybe improve performance of resource lookups.

Related to #23039, but not a solution.

Solution

Change ResourceEntities from a SparseSet to a SparseArray.

SparseArray is pub (crate), so simply exposing it through Deref would cause privacy errors. Instead, remove the Deref impl and add wrapper methods for iter, get, and remove. Change the return types from &Entity to Entity now that they aren't generic.

As background: A SparseArray is a simple Vec<Option<V>>, while a SparseSet is a SparseArray that maps keys to dense indexes, combined with dense arrays of keys and values. That requires a second array operation to find the actual value, but can be much better for memory usage when the values are large, since missing items only take up space for a single index instead of an entire value.

But the values in ResourceEntities are Entity, which are already small! A SparseArray will always be smaller on 64-bit systems, since an Entity is the same size as a usize, and we don't need to store the additional dense and indices arrays. So switching to SparseArray will save a lookup and save memory.

One drawback is that we can no longer use the dense lists to iterate all resources, so methods like iter_resources now need to scan all component ids. I don't expect this to be a problem in practice, though. iter_resources is rarely used, and O(components) isn't all that much worse than O(resources). If it turns out to be an issue, it's also possible to recover this data by querying the IsResource component.

Testing

Inconclusive.

I attempted to run benchmarks, both bevymark as in the linked issue and cargo bench -p benches --bench ecs, but the results were too noisy on my machine to reach any conclusions. And now that I look more closely, we don't have many benches that even use resources!

[alice-i-cecile]

This makes sense to me, and given that perf characteristics are otherwise kind of a wash we might as well save a bit of memory.