Putback MonadTrans instance for postcondition (#57)

* Putback MonadTrans instance for postcondition

GHC's automatic derivation for PostconditionM's MonadTrans instance
introduces a redundant Monad constraint. In order to avoid that, we
do the derivation manually.

* Update quickcheck-dynamic/src/Test/QuickCheck/StateModel.hs

Co-authored-by: Ulf Norell <ulf.norell@gmail.com>

* Remove unneeded import

---------

Co-authored-by: Ulf Norell <ulf.norell@gmail.com>

Author: Arnaud Bailly

quickcheck-dynamic/src/Test/QuickCheck/StateModel.hs

@@ -166,6 +166,9 @@ type instance Realized Identity a = a
 newtype PostconditionM m a = PostconditionM {runPost :: WriterT (Endo Property, Endo Property) m a}
   deriving (Functor, Applicative, Monad)
 
+instance MonadTrans PostconditionM where
+  lift = PostconditionM . lift
+
 -- | Apply the property transformation to the property after evaluating
 -- the postcondition. Useful for collecting statistics while avoiding
 -- duplication between `monitoring` and `postcondition`.