Extensible (polymorphic) variants

[shayne-fletcher]

This code defeats ppx_deriving_yojson:

     type 't a = [ | `A of 't ]
     type 't ext = [ | 't a | `B of 't ]

The problem seems to lie with the ext_of_yojson function.

Here's the error as shown in utop

utop[2]> type 't ext = [ | 't a | `B of 't ][@@deriving yojson];;
Error: This definition has type
         (Yojson.Safe.json -> 't Ppx_deriving_yojson_runtime.error_or) ->
         Yojson.Safe.json -> 't ext Ppx_deriving_yojson_runtime.error_or
       which is less general than
         't0.
           (Yojson.Safe.json -> 't0 Ppx_deriving_yojson_runtime.error_or) ->
           Yojson.Safe.json -> 't0 ext Ppx_deriving_yojson_runtime.error_or