support generics

[teto]

It would be cool if we could have Ghc.Generics support to allow the generation of Convertable's getGet/getPut. It can be very tedious to write this code otherwise.

something like

class Convertable a where
  getGet :: MessageType -> Get a -- ^get a 'Get' function for the static data
  getPut :: a -> Put -- ^get a 'Put' function for the static data

  -- inspired by 
  default getGet :: (Generic a) => MessageType -> Get a
  getGet = genericGetGet

  default getGet :: (Generic a) => MessageType -> Put
  getGet = genericGetPut

[teto]

Here is some attempts master...teto:master assuming the data is an instance of Serialize. But it fails in many cases with errors such as

    
        daemon: DiagCong error too few bytes
        From:       demandInput
    
        CallStack (from HasCallStack):
        error, called at IDiag.hs:449:37 in main:IDiag
    
    or
    
        "Unknown encoding for constructor"
        (Whend dealing with CongInfo)
    
    or
        Error too few bytes