C#: Replace P/Invoke with delegate pointers

- Moves interop functions to UnmanagedCallbacks struct that
  contains the function pointers and is passed to C#.

- Implements UnmanagedCallbacksGenerator, a C# source generator that
  generates the UnmanagedCallbacks struct in C# and the body for the
  NativeFuncs methods (their implementation just calls the function
  pointer in the UnmanagedCallbacks). The generated methods are needed
  because .NET pins byref parameters of native calls, even if they are
  'ref struct's, which don't need pinning. The generated methods use
  `Unsafe.AsPointer` so that we can benefit from byref parameters
  without suffering overhead of pinning.

Co-authored-by: Raul Santos <raulsntos@gmail.com>

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