[Pitch] yielding coroutine accessors (#2697)

* [CoroutineAccessors] Pitch.

* Revisions based on feedback from second pitch thread

- Provide links to the accessors vision
- Use the names `yielding borrow` and `yielding mutate` for the coroutine based accessors
- Add discussion of when to favor these accessors over `get` and `set` under implications for adoption

* update credits

* Update and rename NNNN-yielding-accessors.md to 0474-yielding-accessors.md

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Co-authored-by: Nate Chandler <[email protected]>
Co-authored-by: Stephen Canon <[email protected]>

Author: 3 people