Deprecate async version of withValue and introduce sync version.

[mbrandonw]

We made withValue async on ActorIsolated, but I don't think we really meant to. That makes the method re-entrant, which means inflight operations can overwrite each other.

For example, this test fails when it shouldn't:

func testAsyncWithValue() async {
  let xs = ActorIsolated<[Int]>([])

  let task1 = Task {
    await xs.withValue {
      try? await Task.sleep(nanoseconds: NSEC_PER_MSEC * 100)
      $0.append(1)
    }
  }
  let task2 = Task {
    await xs.withValue {
      try? await Task.sleep(nanoseconds: NSEC_PER_MSEC * 200)
      $0.append(2)
    }
  }
  let task3 = Task {
    await xs.withValue {
      try? await Task.sleep(nanoseconds: NSEC_PER_MSEC * 300)
      $0.append(3)
    }
  }

  await task1.value
  await task2.value
  await task3.value
  let value = await xs.value
  XCTAssertEqual(value, [1, 2, 3])
}