rewrite the tutorial section on boxes

My goal is to explain the underlying concepts first (destructors and then ownership) with plenty of step-by-step examples, so that thoroughly explaining the semantics of mutability, boxes, moves, etc. is a breeze. I'm trying to avoid the comparisons with C++ that were done before, because this should be approachable for people coming from any language. C++ programmers already know these concepts so they aren't the audience that needs to be catered to. Comparisons with other languages can be done in separate documents (like [this one](https://github.com/mozilla/rust/wiki/Rust-for-CXX-programmers)).

This still needs examples for ownership (inherited mutability), owned boxes and managed boxes.