Fix each* methods to return only nil

[makenowjust]

Related pull request: #3815

In fact, I fixed only two (1 and 2) by this change. This change has no effect for codes in many case.

[asterite]

@makenowjust Thank you!

There is an easy way to mark that a method always returns nil:

def each : Nil
  # ...
end

Then you don't have to use nil at the end of the method and also you don't have to remember to return nil in every branch. And this will show up in docs. What do you think?

[makenowjust]

@asterite Good. I'll rewrite it in this way.

[makenowjust]

@asterite Done.

And, I fixed Int#times and some methods to return nil.

[makenowjust]

Hmm...

I think to of Int#upto and Int#downto's arg is bad naming, n is better.

[Sija]

@makenowjust yep, one can use def downto(to n) keep the best from both worlds :)

[asterite]

@makenowjust I fixed the conflicts after I merged another PR but it seems I did it wrong in one place. Let's keep the name as to for now, later we can rename that to n for to, upto and downto