Replace substitution function with a map

It makes using variables with queries more approachable, but some work
still has to be done.
- The type `Subs` should be renamed and moved out of `Schema`, together with
`AST.Core.Value` probably.
- Some kind of conversion should be possible from a user-defined input
type T to the Value. So the final HashMap should have a type like
`HashMap name a`, where a is an instance of a potential typeclass
InputType.

src/Language/GraphQL.hs

@@ -20,7 +20,7 @@ graphql :: MonadIO m
     => NonEmpty (Schema.Resolver m) -- ^ Resolvers.
     -> T.Text -- ^ Text representing a @GraphQL@ request document.
     -> m Aeson.Value -- ^ Response.
-graphql = flip graphqlSubs $ const Nothing
+graphql = flip graphqlSubs mempty
 
 -- | 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