Plan: async Fn and closures

[tmandry]

This is a roadmap item for wg-async. You can view the roadmap here: wg-async roadmap

Write a short doc on the kind of design we expect, and open questions, on: the async Fn traits and async closures.

[tmandry]

I left something here: https://rust-lang.github.io/async-fundamentals-initiative/roadmap/async_closures.html