0.2

CHANGES:

• add Fut.for_list

• add around_task to Pool.create

• add Pool.shutdown_without_waiting

• add Pool.num_tasks

• add Fut.is_done

• add Blocking_queue.size

• add Fut.for_array to easily iterate on an array in parallel

• add Fut.get_or_fail{,_exn}

• perf: limit number of work queues in pool

• perf: use multiple queues and non-blocking work-stealing from them, in pool
  this improves the behavior for many small tasks by reducing contention on
  each queue

• fix: fut: actually run all map/bind callbacks in pool if provided