Implement `@oneOf` input unions

[goto-bus-stop]

What will we need, beyond validation?

graphql/graphql-spec#825

[SimonSapin]

Based on a first skim of the proposed spec:

• A new built-in directive definition
• Some new schema validation rules
• A new rule for input coercion
  • In validation, for literal object values
  • In the execution engine, for variables (even though introspection, the only user of this engine for now, does not use @oneOf)
• A new schema-introspection field