feat: use private static fields to store constant typeParameters where possible

[TimothyMakkison]

Modify source generator to use private static readonly fields to store the typeParameter arrays, saving the allocations for each request.

• Only generates when there are actual arguments, otherwise Array.Empty is used - feat: generate code that uses Array.Empty where possible #1599
• private static fields are generated beside its respective method, this could be moved to the start of the class if requested.
• I've used a HashSet of each member in scope to accidental name collisions. Fields will have names like _typeParameters3 incrementing for each field generated.
• Uses nullable coalescing assignment if one of the method parameters uses type arguments. This is done as we may not be able to access it from outside the method scope making the array initialization invalid.
• It should be safe to initialize and set the static arrays in multithreaded contexts as setting references are thread safe in C#.

Edit: typeof(T) will be different for each usage of T so it isn't possible to have a reusable _genericType array. This can also cause issues for _typeParameter. GenerateTypeParameterExpression ensures that each parameter doesn't contain a generic type parameter, falling back to the current solution if it detects one.

• Tests for the changes have been added (for bug fixes / features)

See #1598

[anaisbetts]

Seems legit, @clairernovotny?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (52151a2) 87.81% compared to head (a54d24e) 87.81%.

Additional details and impacted files

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  Coverage   87.81%   87.81%           
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  Files          33       33           
  Lines        2347     2347           
  Branches      294      294           
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  Hits         2061     2061           
  Misses        207      207           
  Partials       79       79           

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[anaisbetts]

@ChrisPulman Any objections to this PR?

[ChrisPulman]

@ChrisPulman Any objections to this PR?

I am currently in India, but I will check it this evening and merge it if it's fitting to our requirements, thank you.

[ChrisPulman]

All looks good, thank you Tim

[github-actions]

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