v3.0

This thing breaks some API sorta everywhere, but I think it's a nicer thing to work with than it was. I added a configuration struct which allows for finer-grained control of AllUsers/AllGroups, which should be helpful for consumers of the library (I didn't end up using AsRef). I also refactored a bunch of the repetitive code into some traits so that there wasn't quite as much code duplication. I think it's a little more reasonable now, if a bit more complicated.