* include/ruby/ruby.h: constify RBasic::klass and add
  RBASIC_CLASS(obj) macro which returns a class of `obj'.
  This change is a part of RGENGC branch [ruby-trunk - Feature #8339].
* object.c: add new function rb_obj_reveal().
  This function reveal interal (hidden) object by rb_obj_hide().
  Note that do not change class before and after hiding.
  Only permitted example is:
  klass = RBASIC_CLASS(obj);
  rb_obj_hide(obj);
  ....
  rb_obj_reveal(obj, klass);
  TODO: API design. rb_obj_reveal() should be replaced with others.
  TODO: modify constified variables using cast may be harmful for
  compiler's analysis and optimizaton.
  Any idea to prohibt inserting RBasic::klass directly?
  If rename RBasic::klass and force to use RBASIC_CLASS(obj),
  then all codes such as `RBASIC(obj)->klass' will be
  compilation error. Is it acceptable? (We have similar
  experience at Ruby 1.9,
  for example "RARRAY(ary)->ptr" to "RARRAY_PTR(ary)".
* internal.h: add some macros.
* RBASIC_CLEAR_CLASS(obj) clear RBasic::klass to make it internal
  object.
* RBASIC_SET_CLASS(obj, cls) set RBasic::klass.
* RBASIC_SET_CLASS_RAW(obj, cls) same as RBASIC_SET_CLASS
  without write barrier (planned).
* RCLASS_SET_SUPER(a, b) set super class of a.
* array.c, class.c, compile.c, encoding.c, enum.c, error.c, eval.c,
  file.c, gc.c, hash.c, io.c, iseq.c, marshal.c, object.c,
  parse.y, proc.c, process.c, random.c, ruby.c, sprintf.c,
  string.c, thread.c, transcode.c, vm.c, vm_eval.c, win32/file.c:
  Use above macros and functions to access RBasic::klass.
* ext/coverage/coverage.c, ext/readline/readline.c,
  ext/socket/ancdata.c, ext/socket/init.c,
* ext/zlib/zlib.c: ditto.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40691 b2dd03c8-39d4-4d8f-98ff-823fe69b080e