Use the covariant return type feature of C# 9 to allow method overrides to be more specific about their return type. By exposing this information, e.g. for Widget.Clone(), callers will have more information. This allows various casts to be removed as the information is now available within the type system.
This allows the LINQ spelling to be used, but benefits from the performance improvement of the specific methods for these classes that provide the same result.
