Cache `Class::get`

[grovesNL]

Currently gfx spends a significant amount of time calling Class::get.

Similarly to #64, would it be possible to cache these calls somehow? Perhaps there could be a macro that provides this, like class!("NSObject").

We could try to add this to metal-rs directly for the places we need it (example), but we also need this change in other crates (for example, we frequently create NSAutoreleasePool), so it would be great if we could find a way to add this into objc.

cc @kvark @jrmuizel

[SSheldon]

I'm working on a macro that caches the lookup, but @grovesNL it's worth noting you can avoid the lookup entirely and link the class. Like this:

extern {
    #[link_name = "OBJC_CLASS_$_NSObject"]
    static OBJC_CLASS: Class;
}
let cls = unsafe { &OBJC_CLASS }

Unfortunately I can't do that with a declarative macro because there's no way to use inputs to the macro inside the link directive. But it's doable with a procedural macro, and I've got a different macro working in objc-foundation-derive that does this.

[grovesNL]

@SSheldon The new macro looks great. We'll try it out soon, thanks!