Add NonLocalExitFromIterator lint and fix other cops complained by it

[ypresto]

I found bug in our product that aborts iterator unexpectedly, with return statement.

def iterate
    array.each do |item|
        return if item.skip? # oh, no! it returns from "iterate" method!
        ...
    end
end

NonLocalExitFromIterator detects misused return statements in iterator block.
It uses heuristic way to distinguish intended use of return or not:

• No return value.
  Because where next should be used tend to have no return value (but of map).
• Method chain.
  E.g. array.each do |item| ... end and not before_fork do |server| ... end.
• Block with arguments.
  E.g. not foobar.with_lock do ... end.

Cops affect with this bug also found: LiteralInInterpolation and AssignmentInCondition.
UnneededCapitalW is false-positive, but is still easy to use any? instead.

Thanks for great product!

[ypresto]

jshint has /* falls through */ comment to allow case without break.
This would be better to have comment like # non-local exit, but I have no idea how to impl it...

[jonas054]

👍 I like this cop. Just had a few comments.

BTW, the equivalent to /* falls through */ is # rubocop:disable Lint/NonLocalExitFromIterator.

[bbatsov]

Small remark from me - please, capitalize commit messages.

[ypresto]

# rubocop:disable Lint/NonLocalExitFromIterator

I was thinking it is verbose a little, but it is simple enough for something rarely used :)

Thanks for reviewing! Fixed for all comments. Rebased, CHANGELOG added.

[bbatsov]

@ypresto You'll have to rebase again. Sorry about this.

[ypresto]

re-rebased!

[bbatsov]

👍