* symbol.c: remove rb_gc_mark_symbols().
  fstrings refered by static symbols and pinned dynamic symbols
  are registerd by rb_gc_register_mark_object().
  frstring refered by dynamic symbols (not pinned symbols)
  are refered from global_symbols.dsymbol_fstr_hash (Hash object).
  Note that fstrings refered from dynamic symbols must live loger
  than symbol objects themselves because rb_gc_free_dsymbol() uses
  fstring to remove from symbol tables.
  This is why we can not mark fstrings from dynamic symbols.
  This technique reduces root objects for GC marking.
* gc.c (gc_mark_roots): ditto.
* internal.h: ditto.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46772 b2dd03c8-39d4-4d8f-98ff-823fe69b080e