Improve rar memory usage

[majorro]

• Removed redundant BufferPool
• Improved memory usage of UnpackV1:
  • used ArrayPool for window
  • used stackalloc for other array allocations

[majorro]

Benchmark (unpack goes through Unpack29):

using var stream = File.OpenRead("F:\\Rar.rar");
using var reader = ReaderFactory.Open(stream, new ReaderOptions() { LeaveStreamOpen = true });
while (reader.MoveToNextEntry())
{
    if (reader.Entry.IsDirectory)
    {
        continue;
    }

    using var entryStream = reader.OpenEntryStream();
    entryStream.CopyTo(_memoryStream);
    _memoryStream.Position = 0;
    _memoryStream.SetLength(0);
}

Results:

[adamhathcock]

Thank you so much for this. I'm currently doing a lot of this at my current job and meant to update sharpcompress but never got around to it. Doing this hopefully will inspire me to do more!