* string.c (rb_fstring): Use st_update instead of st_lookup +
  st_insert.
* string.c (fstr_update_callback): New callback for st_update.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43968 b2dd03c8-39d4-4d8f-98ff-823fe69b080e