Fixed #5219

Whether current ASTNode#to_s wrap Expressions with parenthesis (or begin/end) is depending on the parent of this. And default wrapping policy is no wrapping. It causes some problems, for example #5219 or see this:

macro foo(x)
  {{ x }}
  {% debug() %}
end

foo(if (true
        false)
      p true
    else
      p false
    end)

Expected output is false of course but this program prints true currently, because {% debug() %} output is:

if true
  false

  p(true)
else
  p(false)
end

So parenthesis is disappeared and program is broken.

I fixed ASTNode#to_s to wrap parenthesis in every time except this Expressions wants indent.