Allow Drop types in ivars

[madsmtm]

Part of #30.

The desire is to allow using Box, Id and WeakId (and at some point Arc, Rc and the corresponding Weak) inside instance variables.

This is accomplished using a new user-facing type IvarDrop<T> (because of current coherence limitations in how Box/#[fundamental] types work).

Usage:

use objc2::foundation::NSObject;
use objc2::declare::IvarDrop;
use objc2::{declare_class, ClassType};

declare_class!(
    struct MyObject {
        ivar: IvarDrop<Box<i32>>,
    }

    unsafe impl ClassType for MyObject {
        type Super = NSObject;
    }

    // Later on, initialize it with `Ivar::write`

    // A `dealloc` method is generated automatically
);