concrete functor and monad transformers

#3SafeHaskell support

All of the modules in transformers can be inferred to be Safe Haskell–compliant by GHC 7.10. It would be useful to mark them as Safe explicitly so that GHC 7.2 can import transformers modules in Safe code. This should just be a matter of putting

#if __GLASGOW_HASKELL__ >= 702
{-# LANGUAGE Safe #-}
#endif

at the top of each module.