Write type-safe GraphQL queries

Given a introspection query response in schema.json, the expression [%graphql {| query { ... } |} ] is rewritten to a 3-tuple (query, kvariables, parse):

• query (type string) is the GraphQL query to be submitted.
• kvariables (type (Yojson.Basic.json -> 'a) -> arg1:_ -> ... -> argn:_ -> unit -> 'a) is a function to construct the JSON value to submit as query variables. The labels and types of argx are extracted from the query. Required variables appear as labeled arguments, optional variables appear as optional arguments.
• parse is a function for parsing the JSON response from the server and has the type Yojson.Basic.json -> < ... >. The shape of the object type corresponds to the returned response.