migrate from hh_json to yojson

Summary:
## What

migrate from Hh_json to yojson.

## Why

More standard, better unicode-handling, one library is better than two.
I hope to delete Hh_json .

Reviewed By: andrewjkennedy

Differential Revision: D93339035

fbshipit-source-id: 52c75c75f58f949fe70e9893d37fc88e6ea2680a

Author: mheiber

hphp/hack/src/client/clientCheck.ml

@@ -875,7 +875,7 @@ let main_internal
     Lwt.return (Exit_status.No_error, telemetry)
   | ClientEnv.MODE_STATS ->
     let%lwt (stats, telemetry) = rpc args @@ ServerCommandTypes.STATS in
-    print_string @@ Hh_json.json_to_multiline (Stats.to_json stats);
+    print_string @@ Yojson.Safe.pretty_to_string (Stats.to_json stats);
     Lwt.return (Exit_status.No_error, telemetry)
   | ClientEnv.MODE_REMOVE_DEAD_FIXMES codes ->
     let%lwt conn = connect args in

hphp/hack/src/utils/core/stats.ml

@@ -33,12 +33,12 @@ let get_stats () = { stats with gc_stat = Gc.quick_stat () }
 
 let update_max_heap_size x = stats.max_heap_size <- max stats.max_heap_size x
 
-let to_json stats =
-  Hh_json.JSON_Object
+let to_json stats : Yojson.Safe.t =
+  `Assoc
     [
-      ("init_parsing_heap_size", Hh_json.int_ stats.init_parsing_heap_size);
-      ("init_shared_heap_size", Hh_json.int_ stats.init_heap_size);
-      ("max_shared_heap_size", Hh_json.int_ stats.max_heap_size);
-      ("master_heap_words", Hh_json.int_ stats.gc_stat.Gc.heap_words);
-      ("master_top_heap_words", Hh_json.int_ stats.gc_stat.Gc.top_heap_words);
+      ("init_parsing_heap_size", `Int stats.init_parsing_heap_size);
+      ("init_shared_heap_size", `Int stats.init_heap_size);
+      ("max_shared_heap_size", `Int stats.max_heap_size);
+      ("master_heap_words", `Int stats.gc_stat.Gc.heap_words);
+      ("master_top_heap_words", `Int stats.gc_stat.Gc.top_heap_words);
     ]

hphp/hack/src/utils/core/stats.mli

@@ -26,4 +26,4 @@ val get_stats : unit -> t
 
 val update_max_heap_size : int -> unit
 
-val to_json : t -> Hh_json.json
+val to_json : t -> Yojson.Safe.t