Respect subscriptions in the executor

After the last commit there were a few places needed to be adjusted to
support subscriptions. This is done and a test case is added.

It is important to implement subscriptions now, because they require
changes to the library API, and they are a big missing part to finish
the executor. When the executor is finished, we can start to provide
more stable API without breaking everything every release. Validation
and introspection shouldn't require much changes to the API; AST would
require some changes to report good errors after the validation - this
is one thing I can think of.

Fixes #5.

tests/Test/StarWars/Schema.hs

@@ -20,7 +20,11 @@ import Prelude hiding (id)
 -- See https://github.com/graphql/graphql-js/blob/master/src/__tests__/starWarsSchema.js
 
 schema :: Schema Identity
-schema = Schema { query = queryType, mutation = Nothing }
+schema = Schema
+    { query = queryType
+    , mutation = Nothing
+    , subscription = Nothing
+    }
   where
     queryType = Out.ObjectType "Query" Nothing [] $ HashMap.fromList
         [ ("hero", heroFieldResolver)