this also changes a few members of NewReference type to make them work in PyRun_String scenario

What does this implement/fix? Explain your changes.

This PR is motivated by an observation of a crash after PyRunString.TestExec2. It calls PythonEngine.Exec, which in case of exception fails to decref PyRun_String result.

I replace PyRun_String return type with NewReference and handle it accordingly.

NewReference instance members replaced with extension methods

In the process of making this change it was discovered, that members of NewReference should be moved to an extension class, because in regular instance members this is passed by mutable reference (e.g. analogous to ref NewReference in C# or NewReference& in C++),
which requires any method calling this member to have a mutable reference to NewReference:

bool TakeReference(NewReference reference)
  // can use `IsNull` here, because `reference` is a mutable parameter-variable
  // but this causes a problem below
  => reference.IsNull; 
NewReference reference = ...;
TakeReference(reference); // <= ERROR: `reference` has to be copied to be passed by value
                          // but we can't simply copy instances of `NewReference`

bool TakeReference(in NewReference reference)
  => reference.IsNull; // <= ERROR: instance member IsNull requires mutable `reference`
                       // but `in` makes it read-only. We could fix it by using `ref` instead,
                       // but that would force us to use `ref` in the caller, its caller, etc

Instead we do this:

bool TakeReference(in NewReference reference) => reference.IsNull();
static bool IsNull(this in NewReference reference)
    // NOTE: `this in` above impossible in regular instance members
  => reference.pointer == IntPtr.Zero;

This would be obsolete one day once dotnet/csharplang#1710 is released.

Does this close any currently open issues?

This might (unlikely) fix this test crash: https://ci.appveyor.com/project/pythonnet/pythonnet/builds/31115525/job/kkciw8xk5jaux5t7

Checklist

• Make sure to include one or more tests for your change