* include/ruby/ruby.h: freeze nil/true/false.
* gc.c (should_be_finalizable): check frozen after checkin FL_ABLE.
* object.c (rb_obj_taint): check
  OBJ_TAINTABLE(obj).
* object.c (rb_obj_freeze): remove immediate_frozen_tbl
   because all of immediate values are frozen. YAY!
* object.c (rb_obj_frozen_p): ditto.
* test/ruby/test_eval.rb: skip instance_variable_set for
  frozen objects.
* test/ruby/test_weakmap.rb: check ArgumentError instead of
  RuntimeError.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47523 b2dd03c8-39d4-4d8f-98ff-823fe69b080e