.NET 8/ C# 12 changes

[TimothyMakkison]

As the release date of .NET 8 draws closer and finalized previews of C# 12 are released, I thought that it would be worth looking at ensuring Mapperly supports the new features. I've compiled some standout issues and potential additions that could be considered.

• Add .NET 8 preview/Roslyn 4.6
  • Add .NET 8/Roslyn 4.6 to the integration tests and upgrade the samples/unit tests to .NET 8 when it is released.
  • Add the integration test to the CI pipeline.
• Consider using the new unsafe accessor feature.
  • See .NET 8 zero overhead private member mapping #600 for accessing private members. I have a working version although it is not ready to be added.
  • It is also possible to access private constructors and bypass init/required members, this could be further explored (perhaps for setting nested init/required properties).
• Look at using CollectionMarshal.SetCount and CollectionMarshal.AsSpan for extremely fast list copying (technically breaking).
• Ensure that type aliasing is supported, unit tests should be added.
• Add support for primary constructors, I'd assume that they have priority over non MapperConstructor marked constructors.

[latonz]

Created #682, #683 and #684. Overview for .NET 8.0 related work: #689

Improvements with CollectionMarshal may need some experiments...

[latonz]

Closing this for now as relevant issues are created. Integration tests are already running on .NET 8.0.
If the experiments with CollectionMarshal bring any actionable results or sth. to discuss feel free to open a new discussion.