Subscription: add @discardableResult to every handle* method

[jlnquere]

Each time we write this kind of code:

subscription.handle(Event.created) { _, _ in
    //
}

The compiler is not happy and give us a warning: Result of call to 'handle' is unused. To fix this, we have to write something like:

_ = subscription.handle(Event.created) { _, _ in
    //
}

But, often it's OK to ignore the result of theses handle methods. That's the purpose of the @discardableResult attribute. So, with this PR, we don't have to do the _ = trick anymore :)

[flovilmart]

Thanks. That's effectively way cleaner :)