User-facing summary

![planning rfc][]

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This is a brief summary of the user-facing changes.

Extend the definition of dyn Trait to include:
- Async functions
- Functions that return -> impl Trait (note that -> (impl Trait, impl Trait) or other such constructions are not supported)
  - So long as Trait is dyn safe
Extend the definition of impl to permit
- #[dyn(box)] and #[dyn(identity)] annotations
TBD