Add SubclassOf trait

[madsmtm]

An idea for making a trait to tell whether a specific class is a subclass of another. Basically, the more generic version of the ClassType trait.

The need arises on NSMeasurement, which is defined in the Objective-C header as:

@interface NSMeasurement<UnitType: NSUnit *>

With this we could define the generic in Rust as:

pub struct NSMeasurement<UnitType: SubclassOf<NSUnit>>;

Unfortunately, automatically declaring it doesn't work, since we have no way of ensuring in the type-system, that Super is not recursive:

pub struct NSObject;
unsafe impl ClassType {
    type Super = NSObject;
}

// Now there's multiple impls of `SubclassOf<NSObject>`

So maybe it'd make sense to add it anyways, and then implement it in extern_class! instead? At least if the problem comes up in other places than NSMeasurement (NSUnit is an "abstract class", meaning it really acts more like a protocol than a class, which is why this case is kinda special).

See also #518.