pack.c: hide associated objects

* marshal.c (to_be_skipped_id): ignore anonymous attributes.
* pack.c (Init_pack): use anonymous ID so that associated objects
  do not appear in the packed result.
* parse.y (rb_make_internal_id): return an anonymous ID for
  internal use.


git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44840 b2dd03c8-39d4-4d8f-98ff-823fe69b080e