Refactor buffer management for ArrayPool

[paulomorgado]

This is mostly a provocatory PR.

Using ArrayPool<byte> for this class has been mentioned around the interwebs.

I'm not sure a drop-in using ArrayPool<byte> is possible, or even a good idea, but here it is.

• Added [Ignore("ArrayPool")] to tests in Tests.cs to skip tests reliant on ArrayPool.
• Commented out ArrayPool-related assertions in Tests.cs.
• Modified buffer length check in ReturnTempBuffer method in RecyclableMemoryStream.cs.
• Replaced custom small pool with ArrayPool in RecyclableMemoryStreamManager.cs.
• Removed small pool fields and related public properties/methods.
• Updated GetBlock and ReturnBlock methods to use ArrayPool.
• Removed small pool state reporting methods.
• Added ArrayPool property to Options class, defaulting to ArrayPool.Shared.

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[benmwatson]

Provocative indeed...not sure how I feel about this one.

Probably the best thing for me to do would be drop it into some test machines running Bing/XAP and see how it compares with the baseline. Those machines are currently being used to validate Bing on .NET 9, but once that's done, this would be an interesting experiment.

[paulomorgado]

Being based on ArrayPool<T> or not, it would be great to have a shared RecyclableMemoryStreamManager that would be used on a parameterless RecyclableMemoryStream.