ruby.h: swap iv_index_tbl and super for struct RClass

* include/ruby/ruby.h (struct RClass): add super, remove iv_index_tbl.
  since RCLASS_SUPER() is commonly used inside while loops, we move it
  back inside struct RClass to improve cache hits. this provides a
  small improvement (1%) in hotspots like rb_obj_is_kind_of()
* internal.h (struct rb_classext_struct): remove super, add
  iv_index_table
* internal.h (RCLASS_SUPER): update for new location
* internal.h (RCLASS_SET_SUPER): ditto
* internal.h (RCLASS_IV_INDEX_TBL): ditto
* object.c (rb_class_get_superclass): ditto
* include/ruby/backward/classext.h (RCLASS_SUPER): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44294 b2dd03c8-39d4-4d8f-98ff-823fe69b080e