[CHANGES]

• Ignore ->done or ->fail on cancelled Futures
  
• Added ->then_done, ->then_fail, ->else_done, ->else_fail
  
• Neaten up naming of fmap* family - provide both long and short
  names for each function
  
• Added Future::Utils::call and call_with_escape
  
• Ensure that dependent futures on subclasses tries to use derived
  futures as prototype if possible