* parse.y: support Symbol GC. [ruby-trunk Feature #9634]
  See this ticket about Symbol GC.

* include/ruby/ruby.h:
  Declare few functions.
  * rb_sym2id: almost same as old SYM2ID but support dynamic symbols.
  * rb_id2sym: almost same as old ID2SYM but support dynamic symbols.
  * rb_sym2str: almost same as `rb_id2str(SYM2ID(sym))` but not
    pin down a dynamic symbol.
  Declare a new struct.
  * struct RSymbol: represents a dynamic symbol as object in
    Ruby's heaps.
  Add few macros.
  * STATIC_SYM_P: check a static symbol.
  * DYNAMIC_SYM_P: check a dynamic symbol.
  * RSYMBOL: cast to RSymbol

* gc.c: declare RSymbol. support T_SYMBOL.

* internal.h: Declare few functions.
  * rb_gc_free_dsymbol: free up a dynamic symbol. GC call this
    function at a sweep phase.
  * rb_str_dynamic_intern: convert a string to a dynamic symbol.
  * rb_check_id_without_pindown: not pinning function.
  * rb_sym2id_without_pindown: ditto.
  * rb_check_id_cstr_without_pindown: ditto.

* string.c (Init_String): String#intern and String#to_sym use
  rb_str_dynamic_intern.

* template/id.h.tmpl: use LSB of ID as a flag for determining a
  static symbol, so we shift left other ruby_id_types.

* string.c: use rb_sym2str instead `rb_id2str(SYM2ID(sym))` to
  avoid pinning.

* load.c: use xx_without_pindown function at creating temporary ID
  to avoid pinning.

* object.c: ditto.

* sprintf.c: ditto.

* struct.c: ditto.

* thread.c: ditto.

* variable.c: ditto.

* vm_method.c: ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45426 b2dd03c8-39d4-4d8f-98ff-823fe69b080e