Roadmap goal: Async fn everywhere

[tmandry]

This issue tracks the steps for setting up a new top-level goal in the async vision roadmap. Please read the instructions for owning a goal.

Goal owner: @tmandry

• [x] Goal has a landing page listing the impact
• [x] Initiatives are defined
• [ ] All initiatives have a landing page
  • [ ] #243: Fundamentals
  • [x] Boxable async functions
  • [x] Async main and tests
• [ ] Goal and all initiatives listed on the roadmap with links to landing pages

Remember, you don't have to do these all at once!

This issue can be closed when all of the above items are complete.