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/StarWars/Data.hs

@@ -66,8 +66,8 @@ appearsIn :: Character -> [Int]
 appearsIn (Left  x) = _appearsIn . _droidChar $ x
 appearsIn (Right x) = _appearsIn . _humanChar $ x
 
-secretBackstory :: ResolverT Identity Text
-secretBackstory = ResolverT $ throwE "secretBackstory is secret."
+secretBackstory :: Resolve Identity
+secretBackstory = throwE "secretBackstory is secret."
 
 typeName :: Character -> Text
 typeName = either (const "Droid") (const "Human")
@@ -161,10 +161,10 @@ getHero :: Int -> Character
 getHero 5 = luke
 getHero _ = artoo
 
-getHuman :: Alternative f => ID -> f Character
+getHuman :: ID -> Maybe Character
 getHuman = fmap Right . getHuman'
 
-getHuman' :: Alternative f => ID -> f Human
+getHuman' :: ID -> Maybe Human
 getHuman' "1000" = pure luke'
 getHuman' "1001" = pure vader
 getHuman' "1002" = pure han
@@ -172,10 +172,10 @@ getHuman' "1003" = pure leia
 getHuman' "1004" = pure tarkin
 getHuman' _      = empty
 
-getDroid :: Alternative f => ID -> f Character
+getDroid :: ID -> Maybe Character
 getDroid = fmap Left . getDroid'
 
-getDroid' :: Alternative f => ID -> f Droid
+getDroid' :: ID -> Maybe Droid
 getDroid' "2000" = pure threepio
 getDroid' "2001" = pure artoo'
 getDroid' _      = empty