• Take a 'return' argument to Future::Utils::repeat; deprecate the
  trial-cloning feature for subclasses
  
• Have ->followed_by/etc... print a warning in void context
  
• Throw an exception when ->followed_by/etc.. code does not yield a
  Future (RT84188)
  
• Ensure that ->needs_all/->needs_any work correctly on a mix of
  immediate and pending Futures (RT84187)
  
• Ensure that ->on_done/->on_fail always return invocant (RT84313)
  
• Ensure that ->on_ready($f) works on cancelled Futures (RT84312)