Should `Handle<T>` implement `DerefMut<T> where T: LocalHandle`?

I can envision uses cases where there are "setter" functions on a resource:

```witx
resource person {
get_name: function() -> string
set_name: function(name: string)
}

new_person: function(name: string) -> person
```

Right now the implementer of this interface can't get a mutable reference to the underlying `T` as `Handle` doesn't implement `DerefMut`.

Thoughts on implementing it and its safety guarantee (should we wrap the underlying resource with a `RefCell` to at least get runtime borrow-checking)?