Allow to yield values after each run of the scope

[Baptouuuu]

Currently the Sink only returns the carried value once the scope and all tasks have finished. This limits the use cases.

A new method on Sink should be added, along with a new method on Continuation, to return a Generator where the scope has the possibility to yield values on each run.

This opens the possibility to build asynchronous systems that stream data.

---

Maybe it should be explored a way for tasks to stream data as well.