WrappedObject's use of UnsafeCell

[martin-hughes]

After #300 I've been trying to convince myself more about the safety of the AML interpreter.

One thing that stands out is the use of UnsafeCell (partly because of the name 😆). So @IsaacWoods I was wondering why you chose to use UnsafeCell within WrappedObject rather than RefCell or RwLock?

It looks like the need for an interpreter "token" is aiming to cause the user to treat the WrappedObject as though it needs to be reader-writer-locked, but without actually enforcing it - is that a correct reading of the code?