FIFO queue of values that uses Futures

 Objects in the Future::Queue class provide a simple FIFO queue the stores
 arbitrary perl values. Values may be added into the queue using the push
 method, and retrieved from it using the shift method.
 .
 Values may be stored within the queue object for shift to retrieve later, or
 if the queue is empty then the future that shift returns will be completed
 once an item becomes available.