Add Tasks.<T>seq(Iterable<? extends Task<?>>, Task<T> resultTask) method

[yborovikov]

Sometimes client needs to run a sequence of tasks, and then one final task that returns the result:

{
  Iterable<Task<Foo>> fooTasks = buildFooTasks(...); // may return empty collection
  Task<Bar> barTask = new BarTask();
  Task<Bar> foosThenBarSeq = Tasks.seq(Tasks.seq(fooTasks), barTask); // will fail if fooTasks is empty
  Task<Bar> foosThenBarSeq = Tasks.seq(fooTasks, barTask); // looks cleaner and less if..then..else blocks
  ...
}

[cpettitt]

Your proposal sounds good to me. I can take this as a pull request, if you're so inclined.

[jodzga]

API in version 2.x has changed so this issue does not apply to current state of ParSeq.