* perf: use ByteMemoryPool
* feat: KPageTableBase/KPageTable new methods to read and write `ReadOnlySequence<byte>`
* new: add IWritableBlock.Write(ulong, ReadOnlySequence<byte>) with default impl
* perf: use GetReadOnlySequence() instead of GetSpan()
* perf: make `Parcel` IDisposable, use `ByteMemoryPool` for internal allocation, and make Parcel consumers dispose of it
* remove comment about copySize
* remove unnecessary Clear()
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