* Added support for enums in Dart.
* Pick non-private names for enum values.
The _ prefix denotes a private member in Dart, so avoid generating enum values starting with this character.
* Properly encode enum values into query paramters.
* Various cleanups.
* Add support for x-enum-values extension.
Use class instead of enum for better ergonomy.
Better generated enum names.
* Fixed test.
* Support enum descriptions.
