Async command launch

[sohocoke]

It is undesirable to keep the task launch operation as a synchronous side-effect of RunOutput#init, since:

1. Xcode thread sanitiser flags a warning for a race condition for the _stderr variable;
2. It will block until the concurrent read to stderr finishes, making RunOutput inappropriate to initialise on a Cocoa app's main thread.

In this PR, RunOutput's initialiser has been turned 'dumb', all mutation eliminated, and the task launch operation relocated to AsyncCommand#launch(complete: @escaping (RunOutput) -> Void)).

Further points:

• To avoid changing the API, CommandRunning#run(_ executable...), which uses AsyncCommand, remains synchronous and waits for #launch to complete, taking on issue 2 above.
• AsyncCommand#launch is a standard asynchronous method, so it makes the presence of PrintedAsyncCommand#onCompletion, which is a completion handler setter, a little awkward.

[kareman]

This is great! Thank you.

My only request is that you make AsyncCommand#launch fileprivate, as I don't see any use for it outside of the run command. The only way a user can get hold of an AsyncCommand instance is with the runAsync command, and then it is always already launched.

[sohocoke]

Changed per previous comment.

I'd like to mention, though, that it would be better for #runAsync to be changed to call #launch, and probably to remove AsyncCommand#init(launch:...) -- as it stands I find the code paths quite confusing.

I stopped short of making these changes since I didn't quite understand the role of the arguments to #runAsync.

[kareman]

Yes, the internal documentation could be made clearer. I will try to improve it before I release the new version for Swift 4.2.