Redo instance variables

[madsmtm]

Using a combination of what's described here and here, now we have:

#[derive(Debug, PartialEq, Eq, Hash, Clone)]
struct MyState {
    my_variable: CanBeAnything,
    // ...
}

declare_class!(
    struct MyClass;

    unsafe impl ClassType for MyClass {
        type Super = NSObject;
        type Mutability = InteriorMutable;
        const NAME: &'static str = "MyClass";
    }

    impl DeclaredClass for MyClass {
        type Ivars = MyState; // Or even `RefCell<MyState>`, depending on if you want fine-grained or coarse locking
    }

    unsafe impl MyClass {
        #[method_id(init)]
        fn init(this: Allocated<Self>) -> Option<Id<Self>> {
            let this = this.set_ivars(MyState {
                my_variable: Default::default(),
                // ...
            });
            unsafe { msg_send_id![super(this), init] }
        }

        #[method(doStuff)]
        fn do_stuff(&self) {
            self.ivars().my_variable.do_stuff();
        }
    }
);

Fixes #414.
Fixes #438.
Fixes #253.
Fixes #513.
Fixes #458.
Replaces / fixes #288.
Replaces / fixes #411.
Replaces / fixes #173.

TODO:

• Make it work fully.
• Documentation.
• More thorough tests.
• Go through all the linked issues, and ensure all items are completed.
• Consider also allowing msg_send_id![this, init] where this: PartialInit in addition to msg_send_id![super(this), init] (since the specific initializer may not have been overridden)
  • Punting this. We'll rather want some sort of special init handling for things that inherit NSObject?