Improved Memory Rental

[martindevans]

Depending on the CommunityToolkit.HighPerformance package and using MemoryOwner for management of buffer rentals.

Replaced almost all uses of ArrayPool, fixed one case where memory was rented but never returned.

Old content:
Introduced SpanRental and MemoryRental, which provide a safer API than ArrayPool.

• Returns a Span or Memory, instead of an array
• Returned buffer is exactly the requested size
• Rental is returned as a disposable struct, if used with using it will always be disposed
• No need to use try/finally everywhere

[martindevans]

Alternatively we could depend on the CommunityToolkit.HighPerformance package and use MemoryOwner. Done this.