WIP Async2

[majocha]

Currently just replaces cancellable.
Features: cold start, multi-start, cancellable, trampolined binds for safe non-tail recursion, exceptions cached in ExceptionDispatchInfo for fast recovery from deep call stacks.

Synchronous executions should be quite fast and low alloc when it's done, but there are some fixes needed:
Currently complex mut rec expressions are not statically compiled, see the nowarn in CheckDeclarations.fs.

Very initial work.

[github-actions]

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@majocha,

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[vzarytovskii]

To avoid confusion - it's not the same "async2" as in runtime-supported async. It's just different re-implementation of async, right?

[majocha]

To avoid confusion - it's not the same "async2" as in runtime-supported async. It's just different re-implementation of async, right?

Nope, no. I wasn't even aware this was a thing until now :) The goal is to possibly replace all the internal use of async and cancellable with one reasonably efficient resumable CE. I went with async2 name after the example in resumable code RFC.

LOL, probably before I ever finish this, the runtime async model will switch to https://github.com/dotnet/runtimelab/blob/feature/async2-experiment/docs/design/features/runtime-handled-tasks.md

I guess this is the fate, always catching up :)