The FFI-using part of Cmd.Play. It uses the FFI because the play monitor wants to directly call to the UI to update the playback indicator, which is much more efficient than going through the whole Ui.Diff thing.

But that means Cmd.Play would import FFI-using modules, which causes a problem for ghci, and, since I want the Cmd.Play functions to be available there, for the REPL as well. So I engage in a hack: the play cmd returns a special Cmd.Play value which contains the arguments for the play monitor thread. The responder treats it as a Done but will call play with the given args.

Actually since I have *Stub modules now I think this hack is no longer needed. But I'll leave it in place for now since it doesn't seem to be hurting anything and it's nice to divide play into low and high level.