delegate! macro, a higher-level way to create Cocoa delegates (e.g. NSWindowDelegate)

[Litarvan]

As @vbo mentioned in #205 the only way to create cocoa delegates such as NSWindowDelegate is to use the objc crate and it's a bit hard.

I thought a macro for this would be useful.

Example with NSWindowDelegate

Without macro :

let mut decl = ClassDecl::new("MyWindowDelegate", class!(NSObject)).unwrap();

decl.add_ivar::<id>("window");
decl.add_method(sel!(windowWillEnterFullScreen:), on_enter_fullscreen as extern fn(&Object, Sel, id));

let cl = decl.register();

let delegate: id = msg_send![cl, alloc];
(*delegate).set_ivar("window", window);

window.setDelegate_(delegate);

With macro :

window.setDelegate_(delegate!("MyWindowDelegate", {
    window: id = window,
    (windowWillEnterFullScreen:) => on_enter_fullscreen as extern fn(&Object, Sel, id)
}));

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This change is 

[vbo]

This is a good change. Although at the time I was thinking more about type-safety of the interface than the amount of typing required to set things up.

[Litarvan]

In fact, i tried looking for such thing, but i saw that NSWindowDelegate is just a protocol, which doesn't really mean something, so i thought making a macro like this for any delegate would be great

[jdm]

Looks like a nice improvement!
@bors-servo r+

[bors-servo]

📌 Commit f65f59b has been approved by jdm

[bors-servo]

⌛ Testing commit f65f59b with merge 965cc88...

[bors-servo]

☀️ Test successful - checks-travis
Approved by: jdm
Pushing 965cc88 to master...

[vbo]

Can we add a test/example code with this? Or maybe update one of the existing examples?

[Litarvan]

I added an example in the doc comment, the same as the one in my first message, but if you want to add something feel free to do it @vbo (i agree this is a small example)