add non-const visitor interface (#22)
add non-const visitor interface (#22)

edwiincao/variant,tempbottle/variant,daniel-j-h/variant,tempbottle/variant,tangrams/variant,daniel-j-h/variant,edwiincao/variant,tangrams/variant,tempbottle/variant,tangrams/variant,daniel-j-h/variant,edwiincao/variant