Prototype mru

[LinerSu]

1. Prototype a simple update cache if the cache is empty.
2. Prototype basic operations:
   • fold: add three examples;
   • expand: add two examples.

[LinerSu]

Details about my implementation, I made mru_region_domain class as a friend class of region_domain (also forward declare as well) because all the fields in the original implementation are private members. I think the friend class is the easier way to design so far.

[LinerSu]

new implementation on PR #60.