We already have an opaque process object so I don't see a reason spawn can't return it as well. Is there a platform
that's difficult to implement spawn to return it?
Making the spec weak to allow an implementation to return it can be a second choice; but I don't like spawn returns
different type of object depending on whether it waits for the process to exit or it runs the process asynchronously.
I thought it would be consistent that spawn returns a process object, from which the exit status can be retrieved (by wait).