[Monads] Generalize the unwrap function

[viboes]

boost/expected/algorithms/unwrap.hpp contains a function that can be generalized to other monads, as it is the case of expected<T>, optional<T> and future<T>.

The first question is in which namespace this function must be defined.
The second is if it is worth defining a default for monads that have only one argument or forward the call to a member function unwrap.