SXmlSerializable shape for fixing Jon Skeet's problems in C# 9.0

IXmlSerializable.cs

/*
  The IXmlSerializable interface assumes the mutability of the type the implements it.
  Now we have readonly structs, that's not a valid assumption. Ideally, there'd be a static
  method that returned a new instance of the type, but you can't have static methods on
  interfaces, because reasons.
*/


[Deprecated]
public interface IXmlSerializable
{
    void WriteXml(XmlWriter writer);
    
    [Obsolete("This seemed like a good idea in 2002, but it causes problems here in 2022")]
    void ReadXml(XmlReader reader);
}

SXmlSerializable.cs

/*
  There's an open C# language discussion for something called Shapes, which are like interfaces
  but much, much better, and they do allow static methods to be declared, which would allow the
  following syntax:
*/

public shape SXmlSerializable<T>
{
    void WriteXml(XmlWriter writer);
    static T ReadXml(XmlReader reader);
}

// Learn more about Shapes at https://github.com/dotnet/csharplang/issues/164