Fix JSON decoder exception when referencing different message’s enum

[mbickers]

Defer JSON enum mapping initialization until first use to avoid an exception when a message references another message’s nested enum.

I added a regression test. Without fix, test fails with

Fatal error: exception File "test/json_enum_initialization.ml", line 126, characters 6-12: Undefined recursive module
Raised at CamlinternalMod.init_mod_field.fn in file "camlinternalMod.ml", line 30, characters 11-49
Called from Ocaml_protoc_plugin__Deserialize_json.map_enum_json in file "src/ocaml_protoc_plugin/deserialize_json.ml", line 197, characters 4-16
Called from Ocaml_protoc_plugin__Deserialize_json.read_value in file "src/ocaml_protoc_plugin/deserialize_json.ml", line 312, characters 25-52
Called from Ocaml_protoc_plugin__Deserialize_json.read in file "src/ocaml_protoc_plugin/deserialize_json.ml", line 333, characters 23-38
Called from Ocaml_protoc_plugin__Deserialize_json.deserialize in file "src/ocaml_protoc_plugin/deserialize_json.ml", line 408, characters 15-24
Called from Ocaml_protoc_plugin__Deserialize_json.deserialize in file "src/ocaml_protoc_plugin/deserialize_json.ml", line 417, characters 20-43
Called from Test__Json_enum_initialization.A.from_json_exn in file "test/json_enum_initialization.ml", line 79, characters 4-119

I hit this when working with the GTFS Realtime protocol: TripUpdate.StopTimeUpdate.departure_occupancy_status references the OccupancyStatus enum nested in VehiclePosition.

Thanks for maintaining this library!

[andersfugmann]

Thanks for the PR. I'll look at this over the weekend. I'd like to look at bit deeper to understand if we can solve the references differently rather than using the lazy keyword. Recursive modules does put some constraints on how other modules can be referenced.

[andersfugmann]

Looking a bit deeper, I think using lazy is the cleanest solution. Thanks for submitting the PR.