Plan: dyn dispatch for AFIT

[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 this topic (#281).

[tmandry]

This is pretty well documented here already: https://rust-lang.github.io/async-fundamentals-initiative/explainer/async_fn_in_dyn_trait.html

We did not identify any blocking issues wrt stabilizing (static) async fn in trait, so closing this.