While the general guidance remains useful, it is not the case that
error.Canceled will always pass across the Group task function boundary.
Remove the too-aggressive assertions and add unit test coverage.
Closes#30096Closes#31340Closes#31358
This function works with a slice of futures and returns the index of a
completed one. This doesn't work very well in practice because it's
either too high level or too low level.
At the lower level we have Io.Batch for doing this kind of thing at the
Operation API layer.
At the higher level we have Io.Select which is a convenience wrapper
around an Io.Group and an Io.Queue.
Replaced by the lockStderr functions of std.Io. Trying to make
`std.process.stderr_thread_mutex` be a bridge across different Io
implementations didn't work in practice.
Now std.Io.Threaded can return error.ConcurrencyUnavailable rather than
asserting. This is handy for logic that wants to try a concurrent
implementation but then fall back to a synchronous one.
