Improve performance of async deserialization

[AArnott]

The problem with async deserialization perf is that async methods cannot pass ref struct around. Passing around ReadOnlySequence<byte> to every single read method kills perf.

In this change, I radically restructure MessagePackReader to create MessagePackStreamingReader which is a ref struct with all synchronous methods except for one that asynchronously fetches more bytes from an arbitrary source. Doing so destroys the ref struct so the caller has to recreate it. I've done what I can to minimize the syntax overhead on the caller, but it's likely to never be as convenient as the MessagePackReader. But it's more convenient and complete than the old async model.

If we use the MessagePackStreamingReader instead of MessagePackReader in synchronous converters, the perf is almost as good, but not quite (at least not yet).
My dream would be that a converter can be written using the new async pattern and have perf that is competitive with the old sync pattern such that implementing a sync converter and an async converter is no longer necessary in any case.

Other benefits of the new model:

• Support for beginning deserialization before all bytes have downloaded even in synchronous mode.
• More APIs that avoid exceptions.
• No mandatory allocation for async deserialization.

Before

BenchmarkDotNet v0.14.0, Windows 11 (10.0.22631.4460/23H2/2023Update/SunValley3)
AMD Ryzen 9 3950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 9.0.101
  [Host]   : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX2
  ShortRun : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX2

Job=ShortRun  IterationCount=3  LaunchCount=1  
WarmupCount=3  

Method	Categories	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Allocated	Alloc Ratio
DeserializeAsArrayInit	array-init,Deserialize	131.6 ns	15.44 ns	0.85 ns	1.00	0.01	0.0095	80 B	1.00
DeserializeAsyncAsArrayInit	array-init,Deserialize	664.5 ns	114.58 ns	6.28 ns	5.05	0.05	0.0257	216 B	2.70
SerializeAsArrayInit	array-init,Serialize	216.1 ns	28.18 ns	1.54 ns	1.00	0.01	-	-	NA
SerializeAsyncAsArrayInit	array-init,Serialize	294.1 ns	38.89 ns	2.13 ns	1.36	0.01	0.0067	56 B	NA
DeserializeAsArray	array,Deserialize	132.8 ns	6.20 ns	0.34 ns	1.00	0.00	0.0095	80 B	1.00
DeserializeAsyncAsArray	array,Deserialize	711.1 ns	415.59 ns	22.78 ns	5.36	0.15	0.0257	216 B	2.70
SerializeAsArray	array,Serialize	211.0 ns	15.74 ns	0.86 ns	1.00	0.01	-	-	NA
SerializeAsyncAsArray	array,Serialize	289.7 ns	9.41 ns	0.52 ns	1.37	0.01	0.0067	56 B	NA
DeserializeMapInit	map-init,Deserialize	210.6 ns	92.10 ns	5.05 ns	1.00	0.03	0.0095	80 B	1.00
DeserializeAsyncMapInit	map-init,Deserialize	644.5 ns	120.17 ns	6.59 ns	3.06	0.07	0.0257	216 B	2.70
SerializeMapInit	map-init,Serialize	280.8 ns	13.08 ns	0.72 ns	1.00	0.00	-	-	NA
SerializeAsyncMapInit	map-init,Serialize	288.8 ns	11.44 ns	0.63 ns	1.03	0.00	0.0067	56 B	NA
DeserializeMap	map,Deserialize	247.9 ns	20.79 ns	1.14 ns	1.00	0.01	0.0095	80 B	1.00
DeserializeAsyncMap	map,Deserialize	751.7 ns	388.72 ns	21.31 ns	3.03	0.08	0.0257	216 B	2.70
SerializeMap	map,Serialize	303.7 ns	84.11 ns	4.61 ns	1.00	0.02	-	-	NA
SerializeAsyncMap	map,Serialize	329.2 ns	90.39 ns	4.95 ns	1.08	0.02	0.0067	56 B	NA

After

BenchmarkDotNet v0.14.0, Windows 11 (10.0.22631.4460/23H2/2023Update/SunValley3)
AMD Ryzen 9 3950X, 1 CPU, 32 logical and 16 physical cores
.NET SDK 9.0.101
  [Host]   : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX2
  ShortRun : .NET 8.0.11 (8.0.1124.51707), X64 RyuJIT AVX2

Job=ShortRun  IterationCount=3  LaunchCount=1  
WarmupCount=3  

Method	Categories	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Allocated	Alloc Ratio
DeserializeAsArrayInit	array-init,Deserialize	138.7 ns	20.46 ns	1.12 ns	1.00	0.01	0.0095	80 B	1.00
DeserializeAsyncAsArrayInit	array-init,Deserialize	527.6 ns	22.42 ns	1.23 ns	3.80	0.03	0.0286	240 B	3.00
SerializeAsArrayInit	array-init,Serialize	212.7 ns	15.80 ns	0.87 ns	1.00	0.00	-	-	NA
SerializeAsyncAsArrayInit	array-init,Serialize	289.1 ns	48.60 ns	2.66 ns	1.36	0.01	0.0067	56 B	NA
DeserializeAsArray	array,Deserialize	133.7 ns	11.03 ns	0.60 ns	1.00	0.01	0.0095	80 B	1.00
DeserializeAsyncAsArray	array,Deserialize	556.2 ns	66.32 ns	3.64 ns	4.16	0.03	0.0286	240 B	3.00
SerializeAsArray	array,Serialize	217.2 ns	35.21 ns	1.93 ns	1.00	0.01	-	-	NA
SerializeAsyncAsArray	array,Serialize	302.2 ns	15.02 ns	0.82 ns	1.39	0.01	0.0067	56 B	NA
DeserializeMapInit	map-init,Deserialize	229.9 ns	19.49 ns	1.07 ns	1.00	0.01	0.0095	80 B	1.00
DeserializeAsyncMapInit	map-init,Deserialize	612.3 ns	19.17 ns	1.05 ns	2.66	0.01	0.0286	240 B	3.00
SerializeMapInit	map-init,Serialize	278.2 ns	117.33 ns	6.43 ns	1.00	0.03	-	-	NA
SerializeAsyncMapInit	map-init,Serialize	301.7 ns	16.10 ns	0.88 ns	1.09	0.02	0.0067	56 B	NA
DeserializeMap	map,Deserialize	245.3 ns	7.66 ns	0.42 ns	1.00	0.00	0.0095	80 B	1.00
DeserializeAsyncMap	map,Deserialize	641.3 ns	15.93 ns	0.87 ns	2.61	0.00	0.0286	240 B	3.00
SerializeMap	map,Serialize	268.9 ns	4.70 ns	0.26 ns	1.00	0.00	-	-	NA
SerializeAsyncMap	map,Serialize	281.9 ns	67.71 ns	3.71 ns	1.05	0.01	0.0067	56 B	NA