The `Schema` has been overhauled to make `Output` monomorphic.
Traversing the `GraphQL` document is handled implicitly while defining
the `Schema`.
The 4th end-to-end test from `graphql-js` has been ported.
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.
