Support subscriptions

This is experimental support.
The implementation is based on conduit and is boring. There is a new
resolver data constructor that should create a source event stream. The
executor receives the events, pipes them through the normal execution
and puts them into the response stream which is returned to the user.

- Tests are missing.
- The executor should check field value resolver on subscription types.
- The graphql function should probably return (Either
  ResponseEventStream Response), but I'm not sure about this. It will
  make the usage more complicated if no subscriptions are involved, but
  with the current API implementing subscriptions is more
  difficult than it should be.

tests/Test/RootOperationSpec.hs

@@ -15,23 +15,23 @@ import qualified Language.GraphQL.Type.Out as Out
 hatType :: Out.ObjectType IO
 hatType = Out.ObjectType "Hat" Nothing []
     $ HashMap.singleton "circumference"
-    $ Out.Field Nothing (Out.NamedScalarType int) mempty
-    $ pure
-    $ Int 60
+    $ ValueResolver (Out.Field Nothing (Out.NamedScalarType int) mempty)
+    $ pure $ Int 60
 
 schema :: Schema IO
 schema = Schema
-    (Out.ObjectType "Query" Nothing [] hatField)
-    (Just $ Out.ObjectType "Mutation" Nothing [] incrementField)
+    (Out.ObjectType "Query" Nothing [] hatFieldResolver)
+    (Just $ Out.ObjectType "Mutation" Nothing [] incrementFieldResolver)
   where
     garment = pure $ Object $ HashMap.fromList
         [ ("circumference", Int 60)
         ]
-    incrementField = HashMap.singleton "incrementCircumference"
-        $ Out.Field Nothing (Out.NamedScalarType int) mempty
+    incrementFieldResolver = HashMap.singleton "incrementCircumference"
+        $ ValueResolver (Out.Field Nothing (Out.NamedScalarType int) mempty)
         $ pure $ Int 61
-    hatField = HashMap.singleton "garment"
-        $ Out.Field Nothing (Out.NamedObjectType hatType) mempty garment
+    hatField = Out.Field Nothing (Out.NamedObjectType hatType) mempty
+    hatFieldResolver =
+        HashMap.singleton "garment" $ ValueResolver hatField garment
 
 spec :: Spec
 spec =