perf: optimise `SyntaxNodeComparer`

[TimothyMakkison]

It appears that SyntaxNodeComparer is run after a file is formated, optimisation here may be impactful

• Prevent null Func<T, T, Result> allocations (still seeing some in the profiler, no idea how to sotp this)
• Prevent boxing of SyntaxTokenList, SyntaxNodeOrTokenList
• Use OrderBy.ToArray instead of OrderBy.ToList
• Create AllSeparatorsButLast to prevent boxing SeparatedSyntaxList<SyntaxNode> and IEnumerable creation

I have some other idea, but I'm focusing on the unhappy path right now.

Benchmark (comparing complexCode)

Before

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
Default_SyntaxNodeComparer	229.1 ms	4.55 ms	7.60 ms	5000.0000	3000.0000	1000.0000	40.53 MB

After

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
Default_SyntaxNodeComparer	187.4 ms	3.51 ms	7.64 ms	3000.0000	1000.0000	31.39 MB

[TimothyMakkison]

Every time Compare was passed as an argument .NET would allocate Func<SyntaxToken, SyntaxToken, CompareResult>. I think this is due to a method group conversion. I prevented this by adding the property Func<SyntaxToken, SyntaxToken, CompareResult> CompareFunc { get; }

This either saves 1.4MB or 1.9MB of memory according to the benchmarks. I'm not sure whats causing this variance.

Timing is inaccurate

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
Default_SyntaxNodeComparer	184.4 ms	2.68 ms	2.97 ms	3000.0000	1000.0000	29.5 MB

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Allocated
Default_SyntaxNodeComparer	190.2 ms	3.68 ms	6.14 ms	187.3 ms	3000.0000	1000.0000	30.05 MB