Wal-Mart will make a big announcement this week: The company, working with consumer-goods manufacturers and a group of universities, will unveil plans to measure the sustainability of every product it sells . In time, sustainability labels could provide us with information about the environmental and social attributes of consumer goods, much as nutrition labels tell us about the content of the foods we eat. I’ve been hearing about Wal-Mart’s sustainability index for about a year, but th