Fungible contract ft_on_transfer refunds user if output value is PromiseOrValue::Promise

[jorgeantonio21]

With the current signature of ft_on_transfer, every time a contract implementation of this method returns a PromiseOrValue::Promise, the latter call on ft_resolve_transfer refunds a user with the total amount that was sent to the given contract. This behavior is implicit and misleading, and it would be more explicit to enforce ft_on_transfer to return a type like U128. Moreover, the current documentation does not address this scenario.