The boxing-arena crate provides a very simply reuse of Box allocation by
keeping a vector of reusable Box allocations that can be used when wanting to
wrap a value in Box.
It would be sometimes easier to introduce boxing-arena in code bases where
Box is already used extensively than to use some other arena crate that
affects type and life-time semantics more drastically.
Basic usage demonstration:
```rust // Prepare a long-lived arena: let mut ba = BoxingArena::new();
// ... per allocation ... :
// Instead of using Box::new directly, we do:
let boxedbigvalue = ba.rebox(big_value);
// Instead of letting Rust drop and deallocate the Box, we do: ba.unbox(boxedbigvalue); ```