Add new api mrb true or false value

The proposal is simple. 

1. defines new API that returns true or false object that have mrb_bool argument. ( 4f9503acd73c5 )
2. uses new API. And mades some optimizations to functions in the core. ( other patches.)

It's for easy to revert the reason why I split small patches.

All patch depends on 4f9503acd73c5. And each patches are individual each other.
So you can revert partly in case you got some bugs.
Even though I checked by mrbtest with no errors, it might I made some degrades.


On my environment, the patched mrbtest shows a few percent code binary size reduction. And a few percent speed up.