Add autoreleasepool_leaking

[madsmtm]

Idea for #86 and/or #30.

Another idea for this:

impl AutoreleasePool {
    const LEAKING: &'static Self = {
        &*ManuallyDrop::new(unsafe { Self::new_leaking() })
    };
}

But that would always yield a static lifetime, which would mean it had to be unsafe somehow!

[madsmtm]

I'm going to close this since #191 added Id::autorelease_return which basically covers all the use-cases this would've had.