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!