SE-300 revisions in response to review (#1302)
SE-300 revisions in response to review (#1302)

* SE-300 revisions in response to review:

- Fix lingering references to `CheckedContinuation<T>` without an error argument
- Include covariant overload of `resume(with:)` for typed `Result`s; this is implemented in
  the standard library but I forgot to include it in the proposal

* Mark proposal as accepted
abertelrud/swift-evolution,apple/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution,natecook1000/swift-evolution,abertelrud/swift-evolution