[SE-0304] Update Structured Concurrency proposal based on feedback from the third review (#1406)

* [SE-0304] Replace `TaskGroup.async` with `TaskGroup.addTask`.

The use of the adjective `async` for creating new child tasks was too
confusing and didn't fit well with the naming guidelines. Instead, use
`addTask`, which makes it clear that we are adding a new task into a
task group (the result of which is available via `next()`).

* [SE-0304] Rename `sleep(_:)` to `sleep(nanoseconds:)`.

* [SE-0304] Split out changes due to review #3

* [SE-0304] Rename TaskPriority.default to TaskPriority.medium

* [SE-0304] Add (Throwing)?TaskGroup.waitForAll

* [SE-0304] Rename `Task.yield()` to `Task.suspend()`

* [SE-0304] Make `Task.sleep(nanoseconds:)` throwing on cancellation

* Re-schedule review

Author: DougGregor