Get rid of ambiguous operator lints from rubocop

* Things like array.each &:method are ambiguous in a sense that
  a simple white space would make it be a bitwise and. Adding
  parens makes it clearer, although I don't like parens so much
* Same thing goes for splates etc.
* #945
Getting rid off useless assignment cops

* Sometimes whole side effect less method calls were just
  leftovers and hence deleted
* #945
Get rid off additional linting cops

* parens for better clarity
* avoid shadowing of variables
* useless public/private modifiers
* variable that starts with _ should not be used
* couple of harder things remaining (eval/Exception)
* #945
Simplifying boolean conditions in text_style_factory

* The boolean conditions there were essentially reversed, this
  gets that reverse into the boolean condition itself
  (boolean logic ftw!)
* Also removed a bit of duplication :)