Improve routing performance with constant stackalloc when matching

[JamesNK]

A possible place to improve matching performance is using a constant stackalloc:

aspnetcore/src/Http/Routing/src/Matching/DfaMatcher.cs

Lines 45 to 48 in 2783b1a

	// First tokenize the path into series of segments.
	Span<PathSegment> buffer = stackalloc PathSegment[_maxSegmentCount];
	var count = FastPathTokenizer.Tokenize(path, buffer);
	var segments = buffer.Slice(0, count);

At the moment the maximum required size is calculated. Could use a constant size up to a limit (64?), and then switch to allocating array or array pool.

Verify any changes with performance tests.

[ghost]

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[mkArtakMSFT]

@JamesNK is this done already?

[JamesNK]

No

https://github.com/dotnet/aspnetcore/blob/main/src/Http/Routing/src/Matching/DfaMatcher.cs#L47-L50

[ghost]

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