To hear entrepreneur Miki Agrawal tell it, her latest business venture has been a long time coming. A really, really long time.Sitting amid piles of sample boxes and mysterious jars containing increasingly fungus-covered plastic wads, she explained that fungi evolved on Earth about a billion years ago and have since happily chugged along decomposing just about everything within their reach. Cut to the middle of the last century, when plastic diapers were invented to address both the convenience and hygiene needs of millions of moms. Because it takes hundreds of years for plastics to break down, the first disposable diaper ever manufactured is still in a landfill somewhere, as are trillions of its closest friends.To make matters worse, Agrawal said, the diaper industry has added…