This `handler` allows to observe the value of the `input` promise, without changing the propagated value. The `output` promise will be resolved with the same value as the `input` promise (the `handler` returned value will be ignored). However, if `handler` throws, `output` is rejected with the new exception. Unlike `finally`, this handler is not called for rejections.
Make sure that the chained value is not copied when `finally` is called for a fulfilled input promise. The value was copied 7 times in the previous version because it was captured in a lambda, which one copied multiple times.
Make sure that the promise state can only be changed by the promise producer (and not consumers) by removing the `fulfill` and `reject` methods from the instance members and introducing a new constructor accepting a resolver lambda. That also means that a promise can't anymore be default constructed.
Add the static `QPromise<T>::resolve` and `QPromise<T>::reject` methods to create synchronously fulfilled or rejected promises, and fix the `qPromise` helper to handle deduced promises (e.g. `qPromise(QFuture<int>()) -> QPromise<int>`).
