Make ListOfBoxes extend AnyVal for Lift 3.

The `ListOfBoxes` class was implemented in #1525. As noted [here](https://github.com/lift/framework/pull/1525/files#r11415211) in Scala 2.10 we can make that extend AnyVal to get some performance optimization.

Since it's 2.10 only, it'll be something we do pointed at Lift 3 instead of 2.6.