Resolve abstract types

Objects that can be a part of an union or interface should return
__typename as string.

src/Language/GraphQL.hs

@@ -16,7 +16,7 @@ import Language.GraphQL.Type.Schema
 import Text.Megaparsec (parse)
 
 -- | If the text parses correctly as a @GraphQL@ query the query is
--- executed using the given 'Schema.Resolver's.
+-- executed using the given 'Schema'.
 graphql :: Monad m
     => Schema m -- ^ Resolvers.
     -> Text -- ^ Text representing a @GraphQL@ request document.
@@ -25,7 +25,7 @@ graphql = flip graphqlSubs (mempty :: Aeson.Object)
 
 -- | If the text parses correctly as a @GraphQL@ query the substitution is
 -- applied to the query and the query is then executed using to the given
--- 'Schema.Resolver's.
+-- 'Schema'.
 graphqlSubs :: (Monad m, VariableValue a)
     => Schema m -- ^ Resolvers.
     -> HashMap Name a -- ^ Variable substitution function.