• Proper behaviour of ->wait_all and ->needs_all on an empty list -
  just return empty immediate done
  
• Proper behaviour of ->wait_any and ->needs_any on an empty list -
  return an immediate failure
  
• Performance improvement to ->done for immediate Future->new->done
  
• Keep a count of pending child futures to avoid quadratic behaviour
  due to linear scan of children every time one completes
  
• Improve efficiency of Future::Utils::repeat and fmap* when trials
  return immediates
  
• Make repeat and fmap* 'return' argument optional by cloning the
  first non-immediate trial
  
• Rework unit tests to avoid dependency on Test::Warn