specialize Preparer a bit, so resourcePrepare can be added

The forall a. in Preparer made resourcePrepare not seem to be usable, so
I specialized a to Bool. Which works for both Preparer Storer and
Preparer Retriever, but wouldn't let the Preparer be used for hasKey
as it currently stands.

Remote/Directory.hs

@@ -6,7 +6,6 @@
  -}
 
 {-# LANGUAGE CPP #-}
-{-# LANGUAGE Rank2Types #-}
 
 module Remote.Directory (remote) where