RFC: Default value validation & coercion
Implements https://github.com/graphql/graphql-spec/pull/793/
* Adds validation of default values during schema validation.
* Adds coercion of default values anywhere a default value is used at runtime
Potentially breaking:
* Remove `astFromValue`
* Changes type of `defaultValue` provided during type configuration from an "internal" to an "external" value.
