[coco] Introduce arity and general arg accessor (#2200)

[coco] Introduce arity and general arg accessor (#2200)

* [coco] Introduce arity and general arg accessor

With this commit, users are able to query the number of arguments over
each coco::Op, and retrive its Nth argument without knowing its details.

Signed-off-by: Jonghyun Park <jh1302.park@samsung.com>
* Fix a typo