Add ToField and ToParamList modules, fix warning in spec

opium.cabal

@@ -62,10 +62,12 @@ library
     exposed-modules:
         Database.PostgreSQL.Opium,
         Database.PostgreSQL.Opium.FromField,
+        Database.PostgreSQL.Opium.ToField
 
     -- Modules included in this library but not exported.
     other-modules:
         Database.PostgreSQL.Opium.Error,
+        Database.PostgreSQL.Opium.ToParamList,
         Database.PostgreSQL.Opium.Oid
 
     -- LANGUAGE extensions used by modules in this package.

test/Database/PostgreSQL/OpiumSpec.hs

@@ -47,7 +47,7 @@ data ScoreByAge = ScoreByAge
 
 instance Opium.FromRow ScoreByAge where
 
-data Only a = Only
+newtype Only a = Only
   { only :: a
   } deriving (Eq, Generic, Show)