Add a proof of concept for using GetAlternateLookup

[bgrainger]

.NET 9 adds support for collection lookups with spans. This permits zero-allocation lookups in the FastEncoder dictionary using segments of an input string.

This PR is a proof-of-concept to start discussion about supporting this more fully when .NET 9 comes out. The performance gains are small, but the reduction in allocations from the lookups is significant. The downside is a significant increase in conditional compilation directives throughout the code.

BenchmarkDotNet v0.14.0, Windows 10 (10.0.19045.5073/22H2/2022Update)
Intel Core i7-10875H CPU 2.30GHz, 1 CPU, 16 logical and 8 physical cores
.NET SDK 9.0.100-rc.2.24474.11
  [Host] : .NET 9.0.0 (9.0.24.47305), X64 RyuJIT AVX2
  2.1.1  : .NET 9.0.0 (9.0.24.47305), X64 RyuJIT AVX2
  PR     : .NET 9.0.0 (9.0.24.47305), X64 RyuJIT AVX2

Method	Job	NuGetReferences	Mean	Error	StdDev	Gen0	Allocated
CountTokens	2.1.1	Tiktoken 2.1.1	8.371 us	0.2182 us	0.3199 us	0.2899	2552 B
CountTokens	PR	This PR	7.977 us	0.0615 us	0.0921 us	-	64 B

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[HavenDV]

Here you use FastEncoder in addition to FastCacheCounts, but you didn't pass new AlternateStringComparer() to it.
Is this implemented by default for regular Dictionary and is only needed for ConcurrentDictionary?
P.S. I'm a bit out of context for now, I'll look into this in the next few days (I saw links on your blog, I'll look into this)

[HavenDV]

Thanks a lot for the research in this direction, it's rare to find people who are passionate about such a low level except for Microsoft employees.

[HavenDV]

Not entirely sure, but also created this issue to explore another possible improvement - Regex.EnumerateSplits - #64