Implemented upb_enumvaldef, for storing information about enumvals.
diff --git a/upb/json_encode.c b/upb/json_encode.c
index 31725cf..b7d6fa9 100644
--- a/upb/json_encode.c
+++ b/upb/json_encode.c
@@ -203,10 +203,10 @@
   if (strcmp(upb_enumdef_fullname(e_def), "google.protobuf.NullValue") == 0) {
     jsonenc_putstr(e, "null");
   } else {
-    const char *name = upb_enumdef_iton(e_def, val);
+    const upb_enumvaldef *ev = upb_enumdef_lookupnum(e_def, val);
 
-    if (name) {
-      jsonenc_printf(e, "\"%s\"", name);
+    if (ev) {
+      jsonenc_printf(e, "\"%s\"", upb_enumvaldef_name(ev));
     } else {
       jsonenc_printf(e, "%" PRId32, val);
     }