parse.y: use rb_fstring() for strings stored in the symbol table

* parse.y (register_symid_str): use fstrings in symbol table
  [Bug #9171] [ruby-core:58656]
* parse.y (rb_id2str): ditto
* string.c (rb_fstring): create frozen_strings on first usage. this
  allows rb_fstring() calls from the parser (before cString is created)
* string.c (fstring_set_class_i): set klass on fstrings generated
  before cString was defined
* string.c (Init_String): convert frozen_strings table to String
  objects after boot
* ext/-test-/symbol/type.c (bug_sym_id2str): expose rb_id2str()
* test/-ext-/symbol/test_type.rb (module Test_Symbol): verify symbol
  table entries are fstrings

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44057 b2dd03c8-39d4-4d8f-98ff-823fe69b080e