Class definition improvements

[madsmtm]

List of improvements that would bring us closer to feature-parity with Swift:

• The ability to safely call superclass methods inside declare_class! (though it's probably not safe to do so outside): Calling super methods safely #455
• Easier init method declaration: declare_class!: Safer initializers #438
• Allow calling declared methods directly (e.g. without having to duplicate them in extern_methods!).
• Automatically generate ivar names.
• Automatically generate helper-module name.
• Require protocol implementors to have a certain mutability (e.g. NSWindowDelegate should require IsRetainable, or perhaps even MainThreadOnly).
  • Partly done in Fix main thread requirements on protocols #509, fully done by Consider removing (non-interior) mutability? #563.
• Store data about methods from virtual protocols in a const to allow safety-checking them as well.
• Allow class methods access to the actual class it was invoked with. Swift solves this with self, could we solve it with an extra argument and perhaps a special attribute on that argument?
• ...