As similar as some of its core products may look from generation to generation, Speck definitely works to improve its already best-in-class designs. The most recent example is CandyShell Edge for iPhone 5 ($40), the newest member of the CandyShell family. Like the others in the line, it combines layers of rubber and plastic in a thoroughly protective, yet slim, body. The big update this time is to the buttons: now, instead of rubber jutting out of…