pack.c: use ivar for associated objects

* pack.c (str_associate, str_associated): keep associated objects
  in an instance variables, instead of in the internal structure.
* string.c (rb_str_associate, rb_str_associated): deprecate.

git-svn-id: svn+ssh://svn.ruby-lang.org/ruby/trunk@44804 b2dd03c8-39d4-4d8f-98ff-823fe69b080e