Added an internal API to get arena from symtab, for Ruby's use.

upb/def.c

@@ -2195,4 +2195,8 @@ size_t _upb_symtab_bytesloaded(const upb_symtab *s) {
   return s->bytes_loaded;
 }
 
+upb_arena *_upb_symtab_arena(const upb_symtab *s) {
+  return s->arena;
+}
+
 #undef CHK_OOM

upb/def.h

@@ -296,6 +296,7 @@ const upb_filedef *upb_symtab_addfile(
     upb_symtab *s, const google_protobuf_FileDescriptorProto *file,
     upb_status *status);
 size_t _upb_symtab_bytesloaded(const upb_symtab *s);
+upb_arena *_upb_symtab_arena(const upb_symtab *s);
 
 /* For generated code only: loads a generated descriptor. */
 typedef struct upb_def_init {