Refere ISEQ BLOCK of binary bytecode to ROM directly. 

I've implemented idea of referring ISEQ BLOCK of binary bytecode to ROM directly. Based on (6) of http://www.csk.com/fukuoka/services/mruby/nxt02.html.

I'm not sure this kind of hack is wellcome, but in my environment, around 8kb of heap can be saved by this change.