fix: StoredValue and Resource panic during cleanup

[luxalpa]

If the value inside a StoredValue implements the drop trait, and that drop causes another read, update or dispose on another StoredValue, we'd get a cryptic panic with already mutably borrowed: BorrowError.
I encountered this issue with a StoredValue<Disposer> with the disposer type returned by as_child_of_current_owner(). Because the Disposer cleans up a reactive sub-scope on Drop, it will try to mutably borrow Runtime::stored_values while it is still mutably borrowed by the function that drops it.

[gbj]

Oh, wow. This is one of those very awkward examples where the scoping of when the BorrowMut handle gets dropped makes a real difference. Nice catch.