`rebind!` macro

[CeleritasCelery]

The rebind macro has this definition:

https://github.com/CeleritasCelery/rune/blob/7136b74386a4586537ffa59c3be4849cb76354fe/src/arena/mod.rs#L96-L102

We are casting the object into a raw form that removes the lifetime. Then we call an unsafe function to create a new object from that raw pointer

Why this is safe

We are not actually changing the &mut reference to Arena. Instead we are releasing it then reborrowing it immutabley. We make sure to shadow the name so the old binding is no longer available.