Async Task Locals

[lewisfm]

What's the motivation for this feature?

It would be helpful to have a way to create task-local global variables.

Describe the solution you'd like

Ideally an API compatible with https://docs.rs/tokio/latest/tokio/macro.task_local.html

Describe the drawbacks, if any

Can't think of any, I'm pretty sure this would just be an improvement.