Refactor UniformTargetRepresentation

[stephen-hawley]

Here's what you need to do:

Make an interface with all the common methods and properties and then make everything implement those interfaces.
The top UniformTargetRepresentation can also implement that interface and do it in terms of code like:

ITargetRepresentation ActualRep {
    get {
        return ((ITargetRepresentation)Library ?? Framework ?? XCFramework);
    }
}
public T SomeInterfaceMethod ()
{
    return ActualRep.SomeInterfaceMethod ();
}