The new guarantees for `Box<T>` across the FFI boundary is welcome. I've been trying to working on trying to pass large data structures (image arrays, for instance) between Rust and C, but have had issues because of doing so without access to FFI-incompatible dynamically-sized things like vectors means that I have consistently run into stack overflow errors at a certain point. Since I believe that `Box<T>` allocates memory on the heap instead, it seems like a step in the right direction for passing that kind of data without needing to resort to `unsafe` on the Rust side of things.
