Generate slices containing all enum values (#350)

It would be useful to generate slices containing all enum values for
each enum type. Then there will be no need to maintain such kind of
slices manually.

Co-authored-by: dikov.e <dikov.e@ct-sg.ru>

Author: Zhenya750

docs/CHANGELOG.md

@@ -30,6 +30,7 @@ When releasing a new version:
 - genqlient now supports subscriptions; the websocket protocol is by default `graphql-transport-ws` but can be set to another value.  
   See the [documentation](FAQ.md) for how to `subscribe to an API 'subscription' endpoint`.
 - genqlient now supports double-star globs for schema and query files; see [`genqlient.yaml` docs](genqlient.yaml) for more.
+- genqlient now generates slices containing all enum values for each enum type.
 
 ### Bug fixes: