Advisories for Cargo/Nano_arena package

Affected versions of this crate assumed that Borrow was guaranteed to return the same value on .borrow(). The borrowed index value was used to retrieve a mutable reference to a value. If the Borrow implementation returned a different index, the split arena would allow retrieving the index as a mutable reference creating two mutable references to the same element. This violates Rust's aliasing rules and allows for memory safety issues …