Move related modules to Language.GraphQL.AST

Fixes #18.

- `Language.GraphQL.Encoder` moved to `Language.GraphQL.AST.Encoder`.
- `Language.GraphQL.Parser` moved to `Language.GraphQL.AST.Parser`.
- `Language.GraphQL.Lexer` moved to `Language.GraphQL.AST.Lexer`.
- All `Language.GraphQL.AST.Value` data constructor prefixes were removed. The
  module should be imported qualified.
- All `Language.GraphQL.AST.Core.Value` data constructor prefixes were removed.
  The module should be imported qualified.
- `Language.GraphQL.AST.Transform` is now isn't exposed publically anymore.

tests/Language/GraphQL/AST/EncoderSpec.hs

@@ -0,0 +1,19 @@
+{-# LANGUAGE OverloadedStrings #-}
+module Language.GraphQL.AST.EncoderSpec
+    ( spec
+    ) where
+
+import Language.GraphQL.AST (Value(..))
+import Language.GraphQL.AST.Encoder
+import Test.Hspec ( Spec
+                  , describe
+                  , it
+                  , shouldBe
+                  )
+
+spec :: Spec
+spec = describe "value" $ do
+    it "escapes \\" $
+        value minified (String "\\") `shouldBe` "\"\\\\\""
+    it "escapes quotes" $
+        value minified (String "\"") `shouldBe` "\"\\\"\""