Add suppport for custom ArrayPools

[jtmueller]

Is your feature request related to a problem? Please describe.
Currently the pooled collections are all hard-coded to use ArrayPool<T>.Shared, but ArrayPool<T> is an abstract class intended to allow for custom implementations. For example, on a resource-constrained device someone might need an ArrayPool of a fixed size, or that pre-allocates arrays or something. At the moment there is no way to get PooledCollections to make use of such a custom ArrayPool.

Describe the solution you'd like
We need a way to optionally specify a custom ArrayPool, and default to ArrayPool<T>.Shared if none is provided. However, passing in an instance of a custom ArrayPool wouldn't work for PooledDictionary or PooledSet, because those types rely on arrays of a private type inaccessible from outside.

Perhaps there's a way to use a generic type parameter to specify the type of a custom ArrayPool, and then create an instance of this type inside the collection code?

[dzmitry-lahoda]

new pooledlist(arraypool x)
{
 pool = arraypool ?? arraypool.shared;
}

I was trying to check how Unity works with fully their own spans-pools for their ECS. But have not found that. I hope arraypool is good abstraction may be implemented on top of Unity primitives.

[jtmueller]

Yes, exactly, to implement this I'll need to add a new ArrayPool<T> optional constructor parameter. But I can only do it for List, Stack, and Queue. Dictionary and HashSet store a private internal type in an array, so it's not possible to pass in an ArrayPool of the correct type for those classes - they will have to keep using the shared pool.

My comment about possibly creating an instance of the custom pool was a bad idea - it's not much of a pool if each instance of PooledDictionary creates its own pool.

[jtmueller]

This is now in place for the three collection types I mentioned.