API incompatibility between target frameworks #50

The definition of CloneFileAsync is different for target frameworks:

The NuGet mechanism requires the API for higher framework being strict superset of lower framework. See more at https://learn.microsoft.com/en-us/dotnet/fundamentals/apicompat/package-validation/compatible-framework-in-package-validator .

In short, the assembly used for run time will be picked by the framework of entry project, but for compile time it will be picked by current project. Such incompatibility will cause MissingMethodException when using in a transitive project targeting netstandard2.0.