Feature parenthesis node

[zhuliquan]

I found there are many operator of adding parentheses in ast/print.go
for example:

expr/ast/print.go

Lines 54 to 56 in 7772ea0

	if _, ok := n.Node.(*BinaryNode); ok {
	return fmt.Sprintf("%s(%s)", op, n.Node.String())
	}

expr/ast/print.go

Lines 91 to 101 in 7772ea0

	if lwrap {
	lhs = fmt.Sprintf("(%s)", n.Left.String())
	} else {
	lhs = n.Left.String()
	}
	if rwrap {
	rhs = fmt.Sprintf("(%s)", n.Right.String())
	} else {
	rhs = n.Right.String()
	}

why not add a specific type ast node for following benefits:
1、make printing string more like the expr inputted by user, i.e. resolve issue: #613.
2、Make the node's responsibilities more single.

[antonmedv]

This is a interesting approach, unfortunately it makes other things more complex. Instead we should just feap thenary nodes in parenthesis if inside binary.