Use an inline array for decompression scratch (#86)

Motivation
----------
This array is small, fixed size, and always created. We can reduce heap
allocations if it's inline within the class rather than a separate
object on the heap.

Modifications
-------------
For .NET 8 only use a fixed array to store the block decompression
scratch buffer.

Results
-------
BenchmarkDotNet v0.13.10, Windows 11
(10.0.22621.2861/22H2/2022Update/SunValley2) 12th Gen Intel Core
i7-1270P, 1 CPU, 16 logical and 12 physical cores .NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  Job-SOJPFQ : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  Job-OKVMCF : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  Job-BRDMES : .NET 6.0.25 (6.0.2523.51912), X64 RyuJIT AVX2
Job-AJAIVB : .NET Framework 4.8.1 (4.8.9181.0), X64 RyuJIT
VectorSize=256

| Method | Runtime | PGO | Mean | Error | StdDev | Ratio | RatioSD |
Rank | Allocated | Alloc Ratio |
|------------ |------------------- |----
|---------:|---------:|---------:|------:|--------:|-----:|----------:|------------:|
| Array | .NET 8.0 | N | 10.77 us | 0.142 us | 0.132 us | 1.00 | 0.00 |
2 | 112 B | 1.00 |
| InlineArray | .NET 8.0 | N | 10.53 us | 0.168 us | 0.157 us | 0.98 |
0.02 | 1 | 80 B | 0.71 |
| | | | | | | | | | | |
| Array | .NET 8.0 | Y | 10.14 us | 0.114 us | 0.106 us | 1.00 | 0.00 |
1 | 112 B | 1.00 |
| InlineArray | .NET 8.0 | Y | 10.11 us | 0.080 us | 0.071 us | 1.00 |
0.02 | 1 | 80 B | 0.71 |

Snappier/Internal/SnappyDecompressor.cs

@@ -9,7 +9,24 @@ namespace Snappier.Internal
 {
     internal sealed class SnappyDecompressor : IDisposable
     {
-        private byte[] _scratch = new byte[Constants.MaximumTagLength];
+#if NET8_0_OR_GREATER
+#pragma warning disable IDE0051
+#pragma warning disable IDE0044
+#pragma warning disable CS0649 // Field is never assigned to, and will always have its default value
+        [InlineArray(Constants.MaximumTagLength)]
+        private struct ScratchBuffer
+        {
+            private byte _element0;
+        }
+
+        private ScratchBuffer _scratch;
+#pragma warning restore CS0649 // Field is never assigned to, and will always have its default value
+#pragma warning restore IDE0044
+#pragma warning restore IDE0051
+#else
+        private readonly byte[] _scratch = new byte[Constants.MaximumTagLength];
+#endif
+
         private uint _scratchLength = 0;
 
         private int _remainingLiteral;
@@ -671,7 +688,12 @@ internal void WriteToBufferForTest(ReadOnlySpan<byte> toWrite)
         internal void LoadScratchForTest(byte[] newScratch, uint newScratchLength)
         {
             ThrowHelper.ThrowIfNull(newScratch);
-            _scratch = newScratch;
+            if (newScratchLength > ((ReadOnlySpan<byte>)_scratch).Length)
+            {
+                ThrowHelper.ThrowArgumentOutOfRangeException(nameof(newScratchLength), "Scratch length exceeds limit");
+            }
+
+            newScratch.AsSpan(0, (int) newScratchLength).CopyTo(_scratch);
             _scratchLength = newScratchLength;
         }