Use stackalloc char&byte arrays for decoding Url values in HttpUtiliy.ParseQueryString

[TrayanZapryanov]

1. Convert UrlDecoder to ref struct
2. If Url is less than 256 characters, do no allocate char&byte arrays
3. Replace substring operation with Span

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[TrayanZapryanov]

Benchmark:

[MemoryDiagnoser]
public class HttpUtilityBenchmarks
{
	private static string UnicodeStr
		=> new string(new[] { '\u304a', '\u75b2', '\u308c', '\u69d8', '\u3067', '\u3059' });
	private static string EncodedUnicodeStr => HttpUtility.UrlEncode(UnicodeStr) + "=" + HttpUtility.UrlEncode(UnicodeStr);

	[Benchmark(Baseline = true)]
	public NameValueCollection ParseQueryString() => HttpUtility.ParseQueryString("https://portal.azure.com/#@testgroup.onmicrosoft.com/dashboard/arm/subscriptions/111111-abac-4067-81ba-5ae054d891b5/" +
	                                                                                "resourcegroups/dashboards/providers/microsoft.portal/dashboards/3456268-8630-60f7-aaac-eca38aca231");

	[Benchmark]
	public NameValueCollection ParseQueryString_Mix() => HttpUtility.ParseQueryString("ReturnUrl=http%3a%2f%2flocalhost%2flogin%2fauthenticate%3fReturnUrl%3dhttp%3a%2f%2flocalhost%2fsecured_area%26__provider__%3dgoogle");

	[Benchmark]
	public NameValueCollection ParseQueryString_Unicode() => HttpUtility.ParseQueryString(EncodedUnicodeStr);
}

Results:
Machine:
BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3593/23H2/2023Update/SunValley3)
11th Gen Intel Core i9-11900K 3.50GHz, 1 CPU, 16 logical and 8 physical cores
.NET SDK 9.0.100-preview.4.24267.66
[Host] : .NET 8.0.5 (8.0.524.21615), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
DefaultJob : .NET 8.0.5 (8.0.524.21615), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI

Before:

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Allocated	Alloc Ratio
ParseQueryString	516.0 ns	7.83 ns	6.94 ns	1.00	0.00	0.1955	-	1.6 KB	1.00
ParseQueryString_Mix	429.1 ns	5.26 ns	4.92 ns	0.83	0.01	0.1931	0.0005	1.58 KB	0.99
ParseQueryString_Unicode	691.2 ns	10.41 ns	9.74 ns	1.34	0.03	0.2689	-	2.2 KB	1.38

PR:

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Allocated	Alloc Ratio
ParseQueryString	489.3 ns	4.92 ns	4.36 ns	1.00	0.00	0.1059	888 B	1.00
ParseQueryString_Mix	338.7 ns	2.76 ns	2.45 ns	0.69	0.01	0.0820	688 B	0.77
ParseQueryString_Unicode	656.1 ns	7.01 ns	6.22 ns	1.34	0.02	0.1717	1440 B	1.62

[MihaZupan]

This is one weird class.
Thanks for taking care of these low-hanging fruit