Fixes #5568

As the issue mentions, this code:

macro expand
  {{ yield }}
  {{ debug }}
end

def foo
  {:foo => 1}
end

expand do
  x = foo[:foo] += 1
  puts x
end

will print this from the debug part of the macro:

begin
  x = __temp_2 = foo
  __temp_2[:foo] = __temp_2[:foo] + 1

  puts(x)
end

The issue is that x = ... is incorrectly printed with the to_s method of an ASTNode because it's either missing parentheses or begin/end so that its value is the last expression (not the first one here). So this PR fixes that.

Eventually, I think it might be better to make this expansion happen after macro code, so showing that macro code would show the original foo[:foo] += 1. In any case, this fix is more general because fixing the to_s should be done regardless of this other possible enhancement.