Write a program that keeps vegetable names and prices in a dictionary as key-value pairs. The program should display a menu that lets the user see a list of all vegetables and their prices, add a new vegetable and price, change the price of an existing vegetable, and delete an existing vegetable and price. The program should pickle the dictionary and save it to a file when the user exits the program. Each time the program starts, it should retrieve the dictionary from the file and unpickle it.