Cop idea: to_enum is the only usage of a method argument

[paulslaby]

Problem description

I just found a bug in my code which could have been found by a Cop.

    def method(args)
      return enum_for(:method, args) unless block_given?
      # never used args in this method
    end 

args argument is really used, but this idiom is very often used and it can tell me, that I used it only in enum_for

Describe the solution you'd like

I'd like to have a cop, which would tell me, that args is unused method argument.
It should complain, if receiver of enum_for is nil of self and the first argument is __method__

I don't know if UnusedMethodArgument, ToEnumArguments or a new Cop should cover it.

It is closely connected to #7753