[WIP] Refactoring Enumerable

__:fire: DO NOT MERGE THIS :fire:__

- [x] 名前が気に食わない

refs:

- https://github.com/megurorb/sit-up/pull/3
- https://github.com/megurorb/sit-up/pull/5
- https://github.com/megurorb/sit-up/pull/6
- https://github.com/megurorb/sit-up/pull/7