Extract method Util#new_range, remove 1st arg to Util#source_range.

I'm working on some refactorings and this one turned out to be quite extensive, so I thought I'd better make a separate PR for it.

In `cop.rb` there's this line added by @yujinakayama:
```ruby
attr_accessor :processed_source # TODO: Bad design.
```

and I make use of that attribute in `Util#new_range` in order to simplify code in cops that use it. Is that ok?