Add rc::Allocated

[madsmtm]

Blocked on #120.

Add a new user facing opaque struct rc::Allocated<T>, which is intended to be used inside Id as Id<Allocated<T>, O>, and can be gotten from msg_send_id![cls, alloc]. This will help users differentiate between allocated and initialized objects.

See also #87 for more discussion about why we want this.

TODO:

• Documentation
• More functionality on Allocated<T>? Postponed
• More functionality on Id<Allocated<T>, O>? Postponed
• Should it be possible to retrieve ivars and initialize them? Postponed to Better class declaration #30
• Consider Allocated<T>(*mut T)

[madsmtm]

We may want to just make it Allocated<T: Message>(*const T), since:

• The ownership doesn't really matter, you shouldn't deref it anyhow
• Deallocation is less efficient if it has to unwrap an Option first (very slight issue)
• Whether we should dealloc after alloc on panic may be runtime specific, and this would allow specialized behaviour for that. Nope, it doesn't differ between runtimes, at least not Apple (old+new) and GNUStep.
• Easier to change afterwards (e.g. just add #[deprecated] type Allocated<T> = Option<Id<NewAllocated<T>, Unknown>>).

[madsmtm]

I'll go with Id<Allocated<T>, O> over Allocated<T>(*mut T) simply because the former requires less code on our end - it's really not that important, both APIs would work fine.