vm_insnhelper.c: use struct rb_constant_entry_t as entry of
RCLASS_CONST_TBL. RCLASS_CONST_TBL has contained VALUE of constant
directly. Now instead rb_const_entry_t is contained in
RCLASS_CONST_TBL, rb_const_entry_t is managed by malloc, and
have not only the value itself but also visibility flag.
This is another preparation for private constant (see
[ruby-dev:39685][ruby-core:32698]).
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@29602 b2dd03c8-39d4-4d8f-98ff-823fe69b080e