transformers: remove instance MonadFix ExceptT and ErrorT

[Henning Thielemann]

I propose to remove the MonadFix instance for the ExceptT transformer, 
because it gives the illusion that it can handle exceptions, which it 
cannot.


The current implementation is:

instance (MonadFix m) => MonadFix (ExceptT e m) where
     mfix f = ExceptT $ mfix $ \ a -> runExceptT $ f $ case a of
         Right x -> x
         Left _ -> error "mfix ExceptT: Left"

You see, that it cannot handle the exception case. Whenever someone 
needs an mfix on ExceptT I strongly suggest that he first handles the 
exception, thus turning (ExceptT e m a) into plain (m a) and then calls 
'mfix' only on (m a).


I further propose to declare the MonadFix instance as non-implementable 
in a way suggested in [1]:

class NoMonadFix m where

instance (NoMonadFix m) => MonadFix (ExceptT e m) where
    mfix = error "unimplementable"

where no instance of NoMonadFix exist and NoMonadFix is not exported. 
Whenever someone tries to call 'mfix' on 'ExceptT e m' he will get the 
type error, that a NoMonadFix instance on 'm' is missing and he will not 
be able to add it.



[1] http://ghc.haskell.org/trac/ghc/ticket/9334#comment:9