full-length nan boxing on 64bit platforms

I extended mruby's current nan-boxing implementation and removed the limitation that it does not work on 64bit platforms. Now it works on x64 as well as x86 and the sizeof `mrb_value` on tthem is exactly 8. One notation is that there is a change in the API of value accessor: getting `p` of `mrb_value` directly via `value.p` is no longer allowed, now you have to get it by `mrb_value_p` macro.