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.

src/Language/GraphQL/Execute/Execution.hs

@@ -3,7 +3,9 @@
 {-# LANGUAGE ViewPatterns #-}
 
 module Language.GraphQL.Execute.Execution
-    ( executeSelectionSet
+    ( coerceArgumentValues
+    , collectFields
+    , executeSelectionSet
     ) where
 
 import Control.Monad.Trans.Class (lift)
@@ -32,10 +34,10 @@ import Prelude hiding (null)
 resolveFieldValue :: Monad m
     => Type.Value
     -> Type.Subs
-    -> Type.ResolverT m a
-    -> m (Either Text a)
-resolveFieldValue result args =
-    flip runReaderT context . runExceptT . Type.runResolverT
+    -> Type.Resolve m
+    -> m (Either Text Type.Value)
+resolveFieldValue result args resolver =
+    flip runReaderT context $ runExceptT resolver
   where
     context = Type.Context
         { Type.arguments = Type.Arguments args
@@ -101,12 +103,12 @@ instanceOf objectType (AbstractUnionType unionType) =
     go unionMemberType acc = acc || objectType == unionMemberType
 
 executeField :: (Monad m, Serialize a)
-    => Out.Field m
+    => Out.Resolver m
     -> Type.Value
     -> NonEmpty (Transform.Field m)
     -> CollectErrsT m a
-executeField fieldDefinition prev fields = do
-    let Out.Field _ fieldType argumentDefinitions resolver = fieldDefinition
+executeField (Out.ValueResolver fieldDefinition resolver) prev fields = do
+    let Out.Field _ fieldType argumentDefinitions = fieldDefinition
     let (Transform.Field _ _ arguments' _ :| []) = fields
     case coerceArgumentValues argumentDefinitions arguments' of
         Nothing -> addErrMsg "Argument coercing failed."
@@ -115,6 +117,7 @@ executeField fieldDefinition prev fields = do
             case answer of
                 Right result -> completeValue fieldType fields result
                 Left errorMessage -> addErrMsg errorMessage
+executeField _ _ _ = addErrMsg "No field value resolver specified."
 
 completeValue :: (Monad m, Serialize a)
     => Out.Type m