Define schema AST.

Large parts of the schema aren't exported publically. They will be made
public during writing the parser.

Fixes #6.

src/Language/GraphQL/AST.hs

@@ -1,7 +1,4 @@
--- | This module defines an abstract syntax tree for the @GraphQL@ language based on
---   <https://facebook.github.io/graphql/ Facebook's GraphQL Specification>.
---
--- Target AST for Parser.
+-- | Target AST for Parser.
 module Language.GraphQL.AST
     ( Alias
     , Argument(..)