Resolve issue where inefficient usage of future::then is common

[camio]

Within a single expression, future::then is frequently used multiple times with relatively fast pieces of code that do not justify the overhead of yielding control to the event loop. This anti-pattern is present in examples within stlab's documentation and is pervasive in actual usage. The extensive usage is likely due to an enhanced clarity of the resulting code.

This task is to come up with a mitigation strategy for this anti-pattern.

Acceptance criteria

• A mitigation strategy has consensus within the stlab team
• The mitigation strategy is executed

Possible solutions

• Modify documentation to warn against this kind of usage
• Incorporate ideas from the Sender/Receiver model which mitigates this by separating task execution from task graph building

[FelixPetriconi]

I see your point.
I use this pattern often to decouple file I/O from the UI, which makes sense from my point of view.
I think it would be good to give the user a hint of how big the task should be that is handled within a single continuation.