Ruby

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