Reported by https://hackerone.com/haquaman

Reported by https://hackerone.com/dkasak

Reported by https://hackerone.com/jpenalbae

Reported by https://hackerone.com/haquaman

Reported by https://hackerone.com/dkasak

As reported by https://hackerone.com/jpenalbae

```
def yie
  yield
end

def bre
  yie {
    1+1
    break
  }
end

p bre #=> display 2, but should be nil
```

allow parenthesised statement as a method argument.
reported [Feature:12686] in bugs.ruby-lang.org fixed in CRuby 2.4.

fix syntax errors with chained assignment with op assign.
reported [Bug:12669] in bugs.ruby-lang.org fixed in CRuby 2.4.

simplify tOP_ASGN rules by command_rhs and arg_rhs rules with
%prec.

reported in [Bug:12402] in bugs.ruby-lang.org fixed in CRuby 2.4

reported by https://gist.github.com/miura1729/53fbd8af889c289a79108e38635b2378
fix proposed by @miura1729 in
https://github.com/miura1729/mruby/commit/b1b7933f7aa950cfb747b06327a0d0340f3e4ff8