Add RefCellValueObserver

[addisoncrump]

RefCell is not Hash (and cannot be Hash), which prevents it from being used with NewHashFeedback. This provides an implementation which allows access to an internal RefCell'd value.

[langston-barrett]

Is it possible to implement

impl<'a, T: Hash> ObserverWithHashField for ValueObserver<'a, RefCell<T>>

instead? Seems like this would acoid duplicating other instances.

[addisoncrump]

Is it possible to implement

impl<'a, T: Hash> ObserverWithHashField for ValueObserver<'a, RefCell<T>>

instead? Seems like this would acoid duplicating other instances.

Sadly no. Since "the developers of the RefCell-providing library may in the future decide to impl Hash for RefCell", we cannot do this. Was the first thing I tried and was so sad to see that compiler error 😅