Box, stack and heap
All values in Rust are stack allocated by default. Values can be boxed
(allocated on the heap) by creating a Box<T>. A box is a smart pointer to a
heap allocated value of type T. When a box goes out of scope, its destructor
is called, the inner object is destroyed, and the memory on the heap is freed.
Boxed values can be dereferenced using the * operator; this removes one layer
of indirection.