More macros

[jgall]

Hi, I made some macros the other day to help out with writing more safe objective-c classes in Rust. Would you have any interest in adding them to your crate?

I wrote them here: https://github.com/jgall/obj_c_rust_test .

Let me know if you like them! If you think they'd be a good addition to your crate I'll submit a PR.