Add suppport for custom ArrayPools

[jtmueller]

Add suppport for custom ArrayPools

[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.