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 <[email protected]>

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:
 

generate/types.go

@@ -147,6 +147,13 @@ func (typ *goEnumType) WriteDefinition(w io.Writer, g *generator) error {
 			val.GoName, typ.GoName, val.GraphQLName)
 	}
 	fmt.Fprintf(w, ")\n")
+
+	// Add slice with all enums.
+	fmt.Fprintf(w, "var All%s = []%s{\n", typ.GoName, typ.GoName)
+	for _, val := range typ.Values {
+		fmt.Fprintf(w, "%s,\n", val.GoName)
+	}
+	fmt.Fprintf(w, "}\n")
 	return nil
 }