text: Title: NotImplementedError when calling toGremlin in FoldedContextField
  Body: Hey guys!
  
  First of all, thanks a lot for this awesome work. I was testing the compiler in combination with Gremlin. The following GraphQL is mentioned in your Readme, but causes a NotImplementedError when trying to generate a Gremlin statement out of it:
  
  ```
  Animal {
          name @output(out_name: "name")
          out_Animal_ParentOf @fold {
              _x_count @filter(op_name: ">=", value: ["$min_children"])
                      @output(out_name: "number_of_children")
              name @filter(op_name: "has_substring", value: ["$substr"])
                   @output(out_name: "child_names")
          }
      }
  ```
  
  Is it a bug or is it just not implemented.
  
  Many thanks!