* include/ruby/ruby.h: add RSTRING_FSTR flag
* internal.h: add rb_fstring() prototype
* parse.y (str_suffix_gen): deduplicate frozen string literals
* string.c (rb_fstring): deduplicate frozen string literals
* string.c (rb_str_free): delete fstrings from frozen_strings table when
  they are GC'd
* string.c (Init_String): initialize frozen_strings table
* test/ruby/test_string.rb: test frozen strings are deduplicated

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42843 b2dd03c8-39d4-4d8f-98ff-823fe69b080e