Add extern_methods! macro

[madsmtm]

Replaces #161.

Syntax:

extern_methods!(
    // `unsafe` here signifies that 
    unsafe impl NSData {
        #[sel_id(new)] // TODO: Doesn't work yet!
        pub fn new() -> Id<Self, Shared>;

        #[sel(length)]
        pub fn len(&self) -> usize;

        // This has a body, and hence is not touched at all
        pub fn is_empty(&self) -> bool {
            self.len() == 0
        }

        #[sel(bytes)]
        fn bytes_raw(&self) -> *const c_void;
    }
);

// Becomes:

impl NSData {
    pub fn new() -> Id<Self, Shared> {
        unsafe { msg_send_id![Self::class(), new].expect("unexpected NULL NSData") }
    }

    pub fn len(&self) -> usize {
        unsafe { msg_send![self, length] }
    }

    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    #[sel(bytes)]
    fn bytes_raw(&self) -> *const c_void;
}

TODO:

• Documentation
• Changelog
• Find a way to allow using msg_send_id and msg_send_bool - should be done in a way that declare_class! also supports such things!
  • Postponed. See Proper bool handling #239 for the msg_send_bool situation. Will be making a #[sel_id(...)] attribute, but that is much easier to do with this merged

[madsmtm]

I've explicitly used it as:

extern_methods!(
    ...
);

Since that allows rustfmt to format the macro contents. Will document this as well! Documented on extern_class! macro.

[madsmtm]

I'm a bit unsure of how big of an improvement this really is. While it is more succinct, error messages get a lot worse, go to source doesn't work in docs, and the unsafety is kind of hidden away.

I did find a bug by converting things to use this: NSArray::objects_in_range was using Vec::as_ptr when it should have been using Vec::as_mut_ptr. So that's good, I guess!

EDIT: With #239 this becomes much more useful, and there's a nice symmetry between this and declare_class, so I think it has value