split_at allows obtaining multiple mutable references to the same data

Affected versions of this crate assumed that Borrow<Idx> 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<Idx> 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 such as writing out of bounds and use-after-frees.

The flaw was corrected in commit 6b83f9d by storing the .borrow() value in
a temporary variable.