`Drop` in `declare_class!` is unsound if the object is deallocated before being fully initialized

[madsmtm]

Realized this in #414 (comment).

Will probably need a drop flag, unless we figure something crazy out, see the above issue for discussion.

There are a few things in std that use similar things to drop flags, so maybe we can take inspiration from those:

• const thread locals

[madsmtm]

Another case where Drop is unsound:
If the object is retained past the drop, or if it is retained from the mutable reference.

The first can be detected by checking the retain count after the drop, like Swift does, but the latter is difficult. Perhaps we can override retain when debug assertions are enabled? Though that likely prevents legitimate use-case.

Also, can weak variables "resurrect" objects that have begun dropping? I really hope not!

[madsmtm]

We can, using autoref-specialization, fairly reliably detect if something explicitly implements Drop, and then act accordingly, see this gist.

[madsmtm]

Cross-referencing that the way we currently deallocate our ivars is be unsound if a dealloc method calls overridden methods #414 (comment)