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/FragmentSpec.hs

@@ -64,12 +64,14 @@ hatType = Out.ObjectType "Hat" Nothing []
         , ("circumference", circumferenceFieldType)
         ]
 
-circumferenceFieldType :: Out.Field IO
-circumferenceFieldType = Out.Field Nothing (Out.NamedScalarType int) mempty
+circumferenceFieldType :: Out.Resolver IO
+circumferenceFieldType
+    = Out.ValueResolver (Out.Field Nothing (Out.NamedScalarType int) mempty)
     $ pure $ snd circumference
 
-sizeFieldType :: Out.Field IO
-sizeFieldType = Out.Field Nothing (Out.NamedScalarType string) mempty
+sizeFieldType :: Out.Resolver IO
+sizeFieldType
+    = Out.ValueResolver (Out.Field Nothing (Out.NamedScalarType string) mempty)
     $ pure $ snd size
 
 toSchema :: Text -> (Text, Value) -> Schema IO
@@ -78,17 +80,15 @@ toSchema t (_, resolve) = Schema
   where
     unionMember = if t == "Hat" then hatType else shirtType
     typeNameField = Out.Field Nothing (Out.NamedScalarType string) mempty
-        $ pure $ String "Shirt"
     garmentField = Out.Field Nothing (Out.NamedObjectType unionMember) mempty
-        $ pure resolve
     queryType =
         case t of
             "circumference" -> hatType
             "size" -> shirtType
             _ -> Out.ObjectType "Query" Nothing []
                 $ HashMap.fromList
-                    [ ("garment", garmentField)
-                    , ("__typename", typeNameField)
+                    [ ("garment", ValueResolver garmentField (pure resolve))
+                    , ("__typename", ValueResolver typeNameField (pure $ String "Shirt"))
                     ]
 
 spec :: Spec