refactor: change GenInfo::is_running

[KowalskiThomas]

What does this PR do?

This PR changes the way GenInfo::is_running works. Previously, it would indicate whether the current coroutine was on CPU; now, it indicates whether the current coroutine or the coroutine it (recursively) awaits is on CPU.
Making that change also allows us to do less work when we check whether the current coroutine is on CPU or not. Because a Coroutine / Generator / GenInfo can only be running if it is not awaiting another Generator, we do not need to compute is_running if we have await != nullptr (and we take await->is_running for the value in that case).

Making that change makes it easier to check whether a Task is currently on-CPU; and allows to do less work when we decide how to unwind asyncio Tasks (cf the changes in TaskInfo which doesn't need the is_on_cpu method that iterates on the await chain anymore).

Note that I checked whether GenInfo::is_running was used in any other way than the one I describe and simplify; it is not, so I do think this change is safe to make as-is.

Note This PR makes sense on its own, but it is in the context of making #198 simpler.

[r1viollet]

LGTM
Ok to iterate on this version as it will make the timeline easier to read.
Though we might want to differentiate these on_cpu cases.

[KowalskiThomas]

For posterity

Though we might want to differentiate these on_cpu cases.

Definitely agree – I initially meant to have two fields, is_running and waiter_is_running or something like that. However, since we would currently only use waiter_is_running, I decided to go for updating is_running – we can always change that in the future if we need to, it's a pretty small and easy change!