Refine top-self as an instance of Object.

I tried to refine top level self as an instance of Object.
I think that this implementation makes `mrb_state` larger but the behavior is similar to CRuby.

It is also a preparation to define singleton methods of top-self, such as `include`.
If @matz accepts this patch, I will send another pull request about top-self's `include`.
(Please refer to [283f8f1](https://github.com/masamitsu-murase/mruby_efi_shell/commit/283f8f19840682c250820ab74418d16a7db6d3e2).)