Actor isolation inheritance. (#2273)

* Extract the isolation-inheritance proposal from my earlier proposal
that combined this awkwardly with some closure isolation features.

* Add missing `isolated` to example

Thanks, Konrad!

Co-authored-by: Konrad `ktoso` Malawski <[email protected]>

* Link to pitch

* Fix typo

Thanks, @arennow!

Co-authored-by: Aaron Rennow <[email protected]>

* Move `@inheritsIsolation` to Future Directions.

* Address review manager feedback.

* Fix a typo in description of Sendable function values.

* Cover distributed actors in generalized isolation checking, and make the
type of `#isolation` more precise.

* Remove incorrect claim about generalizing the distributed-actor-to-actor
conversion using isolated conformances.

* Update the motivating example.

* Prepare SE-0420 for review

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Co-authored-by: John McCall <[email protected]>
Co-authored-by: John McCall <[email protected]>
Co-authored-by: Konrad `ktoso` Malawski <[email protected]>
Co-authored-by: Aaron Rennow <[email protected]>
Co-authored-by: Xiaodi Wu <[email protected]>

Author: 5 people