perf: replace ToImmutableHashSet with manual implementation

[TimothyMakkison]

Replace boxing, Linq and ToImmutableHashSet call with a manual ValueListBuilder version.

• Added a basic contains method because I can't get the built in Span.Contains to work generically with enums 🤷
  • Note that the hashset rarely goes above 4 items in my tests, so we won't be missing out on any vectorisation

Thought I'd already pushed this, I'll go through my backlog and see what else I've missed

Benchmark (timing is inaccurate)

Before

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Allocated
Default_CodeFormatter_Tests	130.5 ms	2.59 ms	6.11 ms	128.2 ms	3000.0000	1000.0000	34.58 MB
Default_CodeFormatter_Complex	261.4 ms	5.16 ms	7.87 ms	260.0 ms	5000.0000	2000.0000	53.04 MB

After

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Allocated
Default_CodeFormatter_Tests	140.1 ms	4.20 ms	12.00 ms	136.2 ms	3000.0000	1000.0000	34.55 MB
Default_CodeFormatter_Complex	265.4 ms	5.20 ms	6.95 ms	265.9 ms	5000.0000	2000.0000	51.99 MB

[TimothyMakkison]

Turns out it was boxing SyntaxKind to do the comparison, fixed the issue and re-ran the benchmarks. Contains doesn't need to be generic, so I made Contains(SyntaxKind)

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Allocated
Default_CodeFormatter_Tests	212.6 ms	9.17 ms	26.30 ms	205.3 ms	3000.0000	1000.0000	34.47 MB
Default_CodeFormatter_Complex	321.9 ms	25.77 ms	75.97 ms	287.8 ms	5000.0000	2000.0000	51.58 MB

Timing is inaccurate, this was done in bed 😅