PostTaskPropagation: Explore .NET or other experience with token propagation

[jyasskin]

https://docs.microsoft.com/en-us/dotnet/standard/parallel-programming/attached-and-detached-child-tasks#cancellation-and-child-tasks says that .NET's cancellation system doesn't provide anything like scheduler.currentTaskSignal for getting the current Task's cancelation token. This may be evidence that it's not needed, or there may be evidence out on the web that .NET developers find this frustrating. It'd be good to figure out which way that evidence goes.

[shicks]

Another important consideration is the challenges faced by https://github.com/legendecas/proposal-async-context

Specifically, I want to make sure we don't expose a half-baked solution that fails to meet some key requirements, such as allowing user-space pooling. At the very least, we need to make sure we learn from past attempts. This is a very hard problem to get right.