Give Query a to_payload() method

[jwodder]

• Give Query a to_payload(&self) -> Payload method with a default implementation that wraps the text of write_graphql() in query($VARIABLES) { ... } to get the full query document

  • The write_graphql() and variables() methods will still be needed for creating batch queries.

• Rename write_graphql() to something like write_query_content() or write_inner_query().