string.c: reduce objects in rb_fstring

* string.c (rb_fstring, rb_str_free): use st_data_t instead of VALUE.
* string.c (rb_fstring): get rid of duplicating already frozen object.
* parse.y (str_suffix_gen): freeze in advance to reduce objects.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42846 b2dd03c8-39d4-4d8f-98ff-823fe69b080e