Refactor cop mixins

Some of the cop mixin modules have too generic name method like `#check`, and also some of them are calling the `#check` defined on another mixin (despite there's no logical dependency between them) by [adapting the method interface forcibly](https://github.com/bbatsov/rubocop/blob/ff167d8f202baf7a68955db0aaf0dc29afb7e7ee/lib/rubocop/cop/mixin/code_length.rb#L17). This is really hard to read and considered bad design.