Rough implementation of execute

The first end-to-end test taken from `graphql-js` passes but this still
needs to be extended to support more general cases.

- `Data.GraphQL.Schema` has been heavily modified to support the
  execution model. More drastic changes are expected in this module.
- When defining a `Schema` ordinary functions taking fields as input are
  being used instead of maps. This makes the implementation of `execute`
  easier, and, arguably, makes `Schema` definitions more *Haskellish*.
- Drop explicit `unordered-containers` dependency. `Aeson.Value`s and
  field functions should be good enough for now.

graphql.cabal

@@ -27,11 +27,10 @@ library
                        Data.GraphQL.Execute
                        Data.GraphQL.Schema
                        Data.GraphQL.Parser
-  build-depends:       base >=4.7 && < 5,
-                       text >=0.11.3.1,
-                       aeson >=0.7.0.3,
-                       attoparsec >=0.10.4.0,
-                       unordered-containers >=0.2.5.0
+  build-depends:       base >= 4.7 && < 5,
+                       text >= 0.11.3.1,
+                       aeson >= 0.7.0.3,
+                       attoparsec >= 0.10.4.0
 
 test-suite tasty
   default-language:    Haskell2010
@@ -41,12 +40,12 @@ test-suite tasty
   ghc-options:         -Wall
   other-modules:       Paths_graphql
                        Test.StarWars
-  build-depends:       base >=4.6 && <5,
-                       aeson >=0.7.0.3,
-                       text >=0.11.3.1,
-                       attoparsec >=0.10.4.0,
-                       tasty >=0.10,
-                       tasty-hunit >=0.9,
+  build-depends:       base >= 4.6 && <5,
+                       aeson >= 0.7.0.3,
+                       text >= 0.11.3.1,
+                       attoparsec >= 0.10.4.0,
+                       tasty >= 0.10,
+                       tasty-hunit >= 0.9,
                        graphql
 
 source-repository head