Rust exercises

Some more Rust exercises. I think that there's certainly something that could be done to improve the grade-school example using named lifetimes, but I'm not sure what the syntax for that would be. /cc @steveklabnik @huonw

I was surprised that the Rust library didn't have a function equivalent to Haskell's [randomR](http://hackage.haskell.org/package/random-1.0.1.1/docs/System-Random.html#v:randomR), but I suppose that's probably because there doesn't seem to be an equivalent to Haskell's [Enum](http://hackage.haskell.org/package/base-4.6.0.1/docs/Prelude.html#t:Enum) typeclass either.